Problems playing MPEG4 AVC(x264) outpu

[DRWin]

Here's an interesting problem.

GoPro footage (4K/60p, MP4) plays perfectly on Windows Media Player and FastStone; also on Windows Classic Media Player MPC-HC.

However, when I do minor editing with Avidemux (sharpen with A-Sharp filter), I have problems.

Saved GoPro footage as MP4 using MP4 Muxer.  I used a 5-second clip as a test.
1. Save as MPEG4 AVC(x264) using MP4 Muxer – 29MB output
   – x264 won't play on Windows Media Player or FastStone.  On another test it would play, but the image started breaking up into pixellated pieces after about one second.  It plays perfectly on Windows Classic Media Player MPC-HC.
For years I have saved with Avidemux using "Save as MPEG4 AVC" using MP4 Muxer with no problems.  Could this new issue be due to an update in the Avidemux program?

2. Save as MPEG4 HEVC using MP4 Muxer – 23MB output
   – x265 plays perfectly on Win Media Player, FastStone, Windows Classic Media Player MPC-HC

3. Save as MPEG4 AVC(x264) using Mkv Muxer – 28 MB output
   – x264 plays perfectly on Win Media Player, FastStone, Windows Classic Media Player MPC-HC

4. Save as MPEG 4 ASP xvid4 using MP4 Muxer – 81MB output
   – plays on Win Media Player, FastStone, Windows Classic Media Player MPC-HC.  But the file size is huge (remember, this is only a 5-second clip) – not practical for a longer clip.

Summary: I have ways to use Avidemux to edit footage and play the result on a variety of players.  But for some reason I no long have the option of saving as MPEG4 AVC (x264) with MP4 Muxer and playing on some common players.  Could this be due to something in Avidemux?

[szlldm]

What version do you use?

[eumagga0x2a]

For years I have saved with Avidemux using "Save as MPEG4 AVC" using MP4 Muxer with no problems.

From the same source at 4K@60fps? In any case a comparison between the last release (2.7.8 ) and the very latest nightly would be interesting.

I wonder whether the upstream change in libavformat in FFmpeg 4.4 which added the btrt atom (bitrate info) to MP4 container can be at fault here. libavformat doesn't calculate the proper max bitrate, thus a player which blindly trusts the info from the container when allocating buffers can end up with a buffer too small for an intra frame, breaking decoding.