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[hmmh]

If I add three subtitles I get a error "Starting new cluster due to timestamp".
If I add less than three subtitles, it doesn't.
This error has existed for several years.
Some TVs have trouble playing the video with this error.

ffmpeg -report -y -i test.mp4 -i tit1.srt -i tit2.srt -i tit3.srt -map 0:v -map 0:a -map 1:s -map 2:s -map 3:s -c copy -c:s srt out.mkv

I'm sending a package of test files...

Another bug:
During encoding of video the time on the screen freezes when the video has subtitles.

frame = 57064 fps = 8057 q = -1.0 size = 530086kB time = 01:12:09.99 bitrate = 1002.9kbits / s speed = 611x

e:\>ffmpeg -report -y -i test.mp4 -i tit1.srt -i tit2.srt -i tit3.srt -map 0:v -map 0:a -map 1:s -map 2:
s -map 3:s -c copy -c:s srt out.mkv
ffmpeg started on 2019-07-11 at 19:43:44
Report written to "ffmpeg-20190711-194344.log"
ffmpeg version 3.4.2 Copyright (c) 2000-2018 the FFmpeg developers

built with gcc 7.3.0 (GCC)
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enabl

e-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-
libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-l
ibtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --en
able-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-l
ibvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libmfx --enable-cuda --enable-cuvid --enabl
e-d3d11va --enable-nvenc --enable-dxva2 --enable-avisynth

libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'test.mp4':

Metadata:

major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.83.100

Duration: 00:02:00.05, start: 0.000000, bitrate: 255 kb/s

Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360 [SAR 1:1 DAR 16:9], 182 kb/s, 29.97 fps,

29.97 tbr, 30k tbn, 59.94 tbc (default)

Metadata:

handler_name : VideoHandler

Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 64 kb/s (default)
Metadata:

handler_name : SoundHandler

Input #1, srt, from 'tit1.srt':

Duration: N/A, bitrate: N/A

Stream #1:0: Subtitle: subrip

Input #2, srt, from 'tit2.srt':

Duration: N/A, bitrate: N/A

Stream #2:0: Subtitle: subrip

Input #3, srt, from 'tit3.srt':

Duration: N/A, bitrate: N/A

Stream #3:0: Subtitle: subrip

Output #0, matroska, to 'out.mkv':

Metadata:

major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.83.100
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360 [SAR 1:1 DAR 16:9], q=2-31, 182 kb/s, 29.

97 fps, 29.97 tbr, 1k tbn, 30k tbc (default)

Metadata:

handler_name : VideoHandler

Stream #0:1(und): Audio: aac (LC) ([255][0][0][0] / 0x00FF), 44100 Hz, stereo, fltp, 64 kb/s (default)
Metadata:

handler_name : SoundHandler

Stream #0:2: Subtitle: subrip (srt)
Metadata:

encoder : Lavc57.107.100 srt

Stream #0:3: Subtitle: subrip (srt)
Metadata:

encoder : Lavc57.107.100 srt

Stream #0:4: Subtitle: subrip (srt)
Metadata:

encoder : Lavc57.107.100 srt

Stream mapping:

Stream #0:0 -> #0:0 (copy)
Stream #0:1 -> #0:1 (copy)
Stream #1:0 -> #0:2 (subrip (srt) -> subrip (srt))
Stream #2:0 -> #0:3 (subrip (srt) -> subrip (srt))
Stream #3:0 -> #0:4 (subrip (srt) -> subrip (srt))

Press [q] to stop, ? for help
[matroska @ 00000000003eaf00] Starting new cluster due to timestamp
frame= 3597 fps=0.0 q=-1.0 Lsize= 3681kB time=00:02:00.00 bitrate= 251.3kbits/s speed= 513x
video:2675kB audio:943kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.730430%
Press any key to continue . . .

[mkver]

Sounds like a duplicate of #7064. As a workaround, you can use -max_interleave_delta 0, but beware: This might increase memory consumption by a lot, especially if at least one of the subtitles is really sparse (like forced subtitles).