Verified with PAF 18.104.22.168
Latest versions of FileZilla have improved Toolbar icon routines. To speed up the icon loading (as they can now be scaled), there is a new icon cache in %LOCALAPPDATA%\FileZilla. Although the cached PNGs are of non-private nature, that behavior is not desired for a portable package.
There is a new setting in fzdefaults.xml to control the cache location.
<Setting name="Cache directory">$USERPROFILE/Documents</Setting>
You might set this in future packages to the inside of the portable application's directory, to not leak any data to the machine and to not leave any traces in the user profile. Furthermore, the cache works better if it is kept with the portable application, plus any possible interactions with installed FileZilla versions will be prevented.