@silverpill @nimda
Thank you for the help guys!
I took a look at my CSV, and it seems that there doesn't seem to be a specific time when accounts are no longer imported: Accounts in the 350+ lines seem to have been imported, but not earlier ones (including accounts on largest mastodon instances)...?
BTW, I tested both CSVs from Iceshrimp (just the account address, one per line), as well as with the Mastodon format (first CSV column being the address, and other columns being booleans about notifications "show boosts", "notify on new posts" and the empty "language" column).