unexpected sample format of mp3

[Mia Sia]

Summary of the bug:
How to reproduce:

PS C:\Users\user\Downloads> ffmpeg -i .\in.wav -sample_fmt s32p -c:a libmp3lame out.mp3
ffmpeg version 2022-05-08-git-f77ac5131c-full_build-www.gyan.dev Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 11.3.0 (Rev1, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enabled --enable-libsoxr --enable-chromaprint
  libavutil      57. 24.101 / 57. 24.101
  libavcodec     59. 27.100 / 59. 27.100
  libavformat    59. 23.100 / 59. 23.100
  libavdevice    59.  6.100 / 59.  6.100
  libavfilter     8. 38.100 /  8. 38.100
  libswscale      6.  6.100 /  6.  6.100
  libswresample   4.  6.100 /  4.  6.100
  libpostproc    56.  5.100 / 56.  5.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, wav, from '.\in.wav':
  Metadata:
    encoder         : Lavf59.23.100
  Duration: 00:00:10.00, bitrate: 1536 kb/s
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s
File 'out.mp3' already exists. Overwrite? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, mp3, to 'out.mp3':
  Metadata:
    TSSE            : Lavf59.23.100
  Stream #0:0: Audio: mp3, 48000 Hz, stereo, '''s32p'''
    Metadata:
      encoder         : Lavc59.27.100 libmp3lame
size=     157kB time=00:00:10.00 bitrate= 128.5kbits/s speed=33.3x
video:0kB audio:157kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.147653%

----

PS C:\Users\user\Downloads> ffprobe -i out.mp3
ffprobe version 2022-05-08-git-f77ac5131c-full_build-www.gyan.dev Copyright (c) 2007-2022 the FFmpeg developers
  built with gcc 11.3.0 (Rev1, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      57. 24.101 / 57. 24.101
  libavcodec     59. 27.100 / 59. 27.100
  libavformat    59. 23.100 / 59. 23.100
  libavdevice    59.  6.100 / 59.  6.100
  libavfilter     8. 38.100 /  8. 38.100
  libswscale      6.  6.100 /  6.  6.100
  libswresample   4.  6.100 /  4.  6.100
  libpostproc    56.  5.100 / 56.  5.100
Input #0, mp3, from 'out.mp3':
  Metadata:
    encoder         : Lavf59.23.100
  Duration: 00:00:10.03, start: 0.023021, bitrate: 128 kb/s
  Stream #0:0: Audio: mp3, 48000 Hz, stereo, '''fltp''', 128 kb/s
    Metadata:
      encoder         : Lavc59.27

[Carl Eugen Hoyos]

This is just how FFmpeg works: You cannot choose the sample format a particular decoder returns, you have to check what it returns. In the past, the sample formats some decoders returned changed (without a version bump) and for a given stream the sample format may change because of compile time options.
Decoding to float is faster on many modern platforms, the mp3 decoder therefore outputs floats.
libmp3lame accepts different input sample formats, some of them certainly faster than others.