Oh yeah, those were very good apps. Exactly how phone apps should be, clear UI, flexible settings with file import/export, etc. It seems it's time to switch, though.

FWIW, I have been using Simple calendar and did not have issues with notifications. I just migrated to Fossify calendar and so far it seems to be sending notifications fine too.
In general notifications can be a pain on OS apps. I managed to pull a few of my friends to use Delta Chat with me. Half of them are not getting notified and it seems we have everything set up the same in the battery settings etc ¯\(ツ)/¯