Missing building slots.

[Yellow Bear]

Not a bug. I feel like these missing building slots are intentional. For example: Nova Trajana Bostra (The capital of Palistine provence) has 5 building slots, not 6. This is because that provence is intended to be weaker.
I did tests, taking the city and upgrading it with multiple factions and the slot is always missing. so ... not a bug.
(More specificaly what I mean by missing building slot: Actual missing slot, not that one is taken by a port or resource or quest building like the damm in Himyar capital.)

[Hat8]

Some towns have fewer slots. It's the way it's always been.

[mah9]

uhm... that is pretty much a bug. the faction that takes the city is irrelevant in this matter. You need 6 slots to have a balanced city, without it you have problems either with public order, food or sanitation, making the whole province utterly worthless.

[Shalomus]

The lack of building slots must be a bug. It doesn't make any sense from a design point of view, considering that only four cities, three in the E. Roman Empire and one in the W. Roman Empire, both provincial capitals and non-capitals, are the only ones missing slots. It is ridiculous to think that those settlements should be permanently weaker short of Creative Assembly trying to add unrealistic and hidden challenge. That is doubtful though, since when this matter was reported to CA, they did not say "just as planned," but responded as if it was a bug. More likely, CA would rather ignore this annoying yet relatively minor bug than put forth the effort to solve it, which is a shame, especially since I'm unaware of how to fix it myself, and I don't think anyone has been able to fix it with simple modding tools and such.

[Hat8]

Originally posted by ForMotherRussia:

uhm... that is pretty much a bug. the faction that takes the city is irrelevant in this matter. You need 6 slots to have a balanced city, without it you have problems either with public order, food or sanitation, making the whole province utterly worthless.

Several cities in France also have this issue. It may be intentional balancing.