Just seen this flying by on terminal output?
[adm_lavLogCallback] 09:39:12-431 [lavc] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead
Should be fixed by [muxer] Populate codecpar, silence lavc deprecated warning (https://github.com/mean00/avidemux2/commit/d2b1f44087b5774f00410db77fb9c560095939cc).
thanks, seems OK now.