I can't see that there's been a recent update yet, but -
On starting up my Samsung Galaxy Tab S4 this morning, I saw there were 38 unread messages !!! First time I'd seen this in a while.
If nothing has been updated, then I can only think that while checking & re-checking my AquaMail settings check-boxes, that when I've disabled & re-enabled (back again), certain settings have been "fixed" ??
Inside Settings / Recent ("settings you changed recently") .... here are the items I've "reset" by clicking off/on again.
Message notifications - Notify about - All unread / re-enabled
Smart watch enhancements - / re-disabled
Android 7+, Android Wear, Pebble, Garmin, Samsung Gear
Notify about ongoing operations - / re-disabled
Progress notifications for loading of messages/attachments and sending messages
My wife's Samsung Galaxy Tab S4 still has only 6 on the icon badge (she has plenty more unread, than 6). I might do the same on her tablet and enable / disable etc, to see if it fixes her issue too.
Having sent a number of emails from another device back to my Samsung Galaxy Tab S4 - I see the counter has changed appropriately on the AquaMail icon.