I do not know how to regenerate WORDS lists, but even today, two days after the creation of 肉 and 肉羹, I did not see either word in the WORDS list looking for 羹. It seems that the word in DICT is never shown in WORDS, so I do not immediately know from WORDS if I added the single characters of a word to my flashcards when I added a more syllable word. [It would be a good idea to be able to check if all characters used in flashcard words are also in flashcards, or even to create them automatically - I do that from time to time with Excel]Did you regenerate that WORDS list since you added that word to flashcards? (exited / reopened that definition screen) It won't reload if you simply switch tabs without moving to a new definition - in general we don't expect flashcards to change that much and we'd rather save time / battery life by not reloading that list more often than we need to.
Update: now it's Beta 5, fixing a newly introduced bug in our Storage Access Framework support (see below).
"Unknown Volume" usually means that your SD card is inaccessible - this would also make it impossible to run Pleco from it if it were already installed on that. Any chance your SD card might have come loose, or been unmounted by some third party app (e.g. for file sharing)?
With SAF, if you tap on the options menu at the top right corner of the screen there's an option to show 'advanced devices' that should let you access everything on your system.