ffmpeg inside while loop with find unintentionally alters iteration variable

Can someone please explain to me what is happening here? This is what I have reduced my situation down to:

# make 20 test gifs out of the same source file.
for i in {1..20}; do cp -p ../some-random-source-file.gif "${i}.gif"; done

# grab, then process them.
while read f; do echo "→ $f"; ffmpeg -i "$f"  -y  -loglevel quiet  "same.mp4"; done < <(find .  -maxdepth 1  -type f  -iname "*.gif"  -printf "%f\n")
→ 11.gif
→ .gif
→ 9.gif
→ .gif
→ 14.gif
→ 9.gif
→ 0.gif
→ 13.gif
→ 7.gif
→ 5.gif
→ 2.gif
→ .gif
→ 3.gif
→ 0.gif
→ 16.gif
→ .gif
→ 8.gif
→ 8.gif
→ .gif
→ 4.gif

I am trying to process all gifs in a directory with mixed files (gifs and non-gifs). But for some reason, as soon as I add the ffmpeg step, the content of the $f variable is sometimes cut off at the beginning. --- Extra info: * I'm using process substition because I am also logging the files that didn't work. if ! ffmpeg …; then FAILED+=("$f"); fi * I'm also generating the new filename via string substitution ${f%.gif}.mp4, but … turns out: that part isn't even relevant for the problem to occur. * bash version 5.1.16 I understand that the process substitution [can cause timing issues](https://unix.stackexchange.com/a/190857/278372) – but then why does it randomly cut off the variable _at the beginning_ and not at the end? And how is anyone to use this construct if it is so unreliable? How else am I to do this?