Weird, both the official Reddit app and Lunar for Lemmy also have a custom icon feature and they don’t seem to be having the same issue.

Even after restarting my phone in order to fix it, Voyager “forgot” its icon again after simply closing and reopening it. I did not even change the icon.

EDIT: after some experiments I found out that this problem only seems to occur when the “O.G.” is used. Is the iOS app perhaps simply missing a small version of that icon?