Why does find ....-execdir rename miss files? zsh

s/_720p// only replaces the first occurrence of _720p with nothing, so it would rename a foo_720p_bar_720p file to foo_bar_720p.

You need to add the g flag to substitute every occurrence: s/_720p//g, but even then, a file called foo_72_720p0p for instance only has one occurrence of _720p but would be renamed to foo_720p once that occurrence has been removed.

You'd need while (s/_720p//g) {} in that case to retry the substitution until it doesn't find anything.

Also note that some perl-based rename implementations have a -d option to only operate on the base name of the files which would save having to run one rename per file¹

find . -depth -name '*_720p*' -type f -exec rename -d ' while (s/_720p//g) {}' {} + 

Note that zsh has a batch renamer built-in in the zmv autoloadable function:

autoload -Uz zmv zmv '(**/)(*_720p*)(#q.)' '$1$2:gs/_720p//' 

Would replace all the occurrences. To handle the _72_72_720p0p0p cases:

zmv '(**/)(*_720p*)(#q.)' '$1$2:fs/_720p//' 

Where the f causes the following s modifier to be repeated until there's no modification.

^{¹ But would not benefit from the safety improvements of -execdir. Note that some find implementations support {} + for -execdir as well, though it still needs to run at least one rename per directory.}