It happened to me in late game while playing as Gwined, then as Mide.
Whenever I conquer new settlements in England, I earn less and less. It's not just large towns (which would be normal) but also farms and quarries. When I look at individual buildings descriptions, they are all supposed to give you extra money; none of them are unknown technologies either. So I had to gradually dismantle my armies and raise the taxes to maximum, causing uprisings everywhere; I can barely afford to support a single largish army and 2 tiny ones. I own all Ireland and big chunks of England; I built as many upgrades as possible, but the profits are simply not there.
Another apparent bug is that I'm no longer allowed to enact decrees.
So many issues with this game, it's anticlimactic. All I can do is watch the Normauns conquer everything while I struggle with bloody rebellions everywhere.