Yes, precisely. SPQR captured Arretium, Segesta and Mediolanum from the Macedonians. They are now the second largest Roman faction after myself and north west Italy is now purple.
Well I'll be !@#$%! Been playing this game for almost ten years and because I've never seen the SPQR capture a city, I just assumed they were forbidden.

This happens a lot I've noticed.
All too often, and it's especially frustrating when it's an ally who you are propping up. If the AI would just get aggressive, they would be able to fend for themselves. But I did have an alliance in my last Brutii campaign with Macedonia that was notable for several reasons:

...the alliance was in effect for over 100yrs. even with a long common border
...after getting and granting military access so I could assist in their war with Thrace, they pulled off a naval landing in Anatolia after eliminating Thrace (I had fully expected the inevitable backstab)
...and they remained aggressive as hell throughout the campaign

I don't know if it's because the town is full and there are no accessible squares next to the town and therefore it will teleport new units beyond the blockade
I would assume this to be the case, though why the distance is so great is puzzling.

whilst I remain a neutral paymaster to the Macedonians. The Senate are average in standing towards me but the mob really hates me.
Isn't it fun to play god?