I wouls almost be inclined to say it's linked to the Reforms - in my current Pontus campaign the bodyguards suddenly went Cappadocian while the first Royal Palace was still only half done, and the Roman stacks traipsing around Greece seemed to start getting their first post-Marian units soon afterwards. I'll be damned if there isn't a connection.
More conclusively, in my older Egyptian campaign (started before the 1.5 update though, assuming that counts one way or the other) where naturally the Nile cities went max level fairly early on, the generals still seem to be driving around in chariots despite multiple maxed-out palaces...
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