all the map and minimap icons for flight masters, boats/zeppelins, auctioneers, bankers, inns, mailboxes, etc. are nowhere to be found on my map or minimap. I've tried uninstalling and reinstalling the addon, and they show up for a second or two and immediately disappear. I logged onto a different character on the same realm and everything showed up.

Of course u did disabled all other addons and checked this only with Carbonite + Modules enabled? Try resetting your carbonite profile, maybe you did messed with settings on this char.

I figured it out!! After reinstalling WoW Classic and having the issue persist, I tried untracking all my quests one by one, and found that "Crystals of Power", a quest in Un'Goro Crater, was causing all the map POI not to show up.

Ok, i know what might be the problem with this Quest. Map Icons and Blobs number is limited and this quest has so many places on map that it takes those from Map Icons... Will check this in future.

Thantks for feedback.

I've reset the Carbonite profile, and tried it on every character on every server with all other addons enabled and disabled. It only happens on my 48 gnome rogue. It's like after 5 seconds of being logged in it auto-unchecks the "Show POI" box in the maps options, but I look and it's still checked. And checking and unchecking it doesn't do anything.