When I attacked an enemy city, this was the victory screen:

That's 84 million mana lost... for losing his sovereign. Seems excessive! Additionally (and probably somehow relatedly) I did not capture the city, as shown here:

This is immediately after winning the battle. Note that there is a hero without all of their health missing garrisoned in the city. How is that possible?

Here is what I believe the problem is: I defeated the lone hero prior to attacking the city and he was sent into the city. However, the city was full (two full pages of units before I attacked), so the fallen hero was somehow stored and did not show up in the siege battle. When I won the fight, the city did not change sides because it still had that wounded hero inside. The extreme mana loss may be somehow related to the bug. This is all just my theory of course, hopefully this info helps.