[Description/Steps to Reproduce] From time to time, I was able to witness some battles, which contained Fortifications/Cities' Districts tiles. I guess they are technically not a Siege, since no City was directly involved in those battles. Like the screenshot below:

Therefore this causes some issues with combat system. Here are some examples:

When trying to attack enemy Units on fortified tiles with my Mortars, enemy Units were able to maintain +6 Combat Strength from Fortification. Plus Walls/Fortifications apparently were showing numerical errors in terms of health and its Strength.

Similarly, when attacking enemy Units with my Janissaries, enemy Units were also able to maintain +6 Combat Strength from Fortification, while my Janissaries were unable to gain +4 Combat Strength from Siege Mastery¹.

[Reproducibility] Unclear. Since Mortars appear quite late game, and battle conditions like this is relatively rare, I was unable to reproduce from other saves. I tried to do something recently, but failed to do so due to constant late-game crashes and enemy Units retreating.

[Expected Behaviour] I think consistency is needed. Either provide benefits to both attackers and defenders or none at all. For instance, Units get +6 Combat Strength from Fortifications, but those Fortifications can be destroyed by Mortar fires. Plus, if the former were to be fixed in this way, Janissary should retain Siege Mastery, when attacking Fortifications,

[Notes] I believe it is indeed a bug just by looking at the numerical errors. However I think the blue print(or goal) of this system is a bit complicated. If in non-siege battle Fortifications can be destroyed, when are they going to be repaired? Will they retain damages for certain turns? Or will Fortifications be invincible to Mortars when not in Siege? If that were a case, what would happen to one tile Fortification(like Strong hold or castle)? Many questions follow.

1) Not sure if the name and effect are 100% correct. Janissary has special ability, which gives more Combat Strength when attacking in Siege.