Depends on the app. Running snap info foo (replacing foo with the snap name) may give you contact details, in which case click on that and post there. Otherwise, look at the author, if it looks like the snap author is the app author, then report the bug on their project page (e.g. if they use GitHub then report on GitHub Issues), if it looks like the snap author is not the app author, then report a bug here under the ‘snap’ category and tag the snap author if possible (e.g. if reporting a bug against Corebird tag diddledan).
If in doubt, and sometimes also when not in doubt, post on the project bugtracker and post here in the ‘snap’ category as well.