With ffmpeg, as far as I know, it was never even tried so possibly never worked.  The native image sequence is the original implementation by HV (1996) which occurred before the initial release of ffmpeg in the year 2000.
may be it always was like this? (but may be you can spin VM with older distro for testing) 
I can't determine the version where the problem with the image
sequences starts. Appimage 20210930 already has the problem. Previous
ones don't work for me in Arch, giving a segv and an empty dump file.\

I am still testing here with little reportable progress.