Slitherine

look like a bug
There is no teritories for Rome.
at the previous turns most of Etrusque lands was Rome.
this turn no more
there are some roman army but that all.

My guess is Rome agreed to cede all it's territories to Etrusque. It wasn't an "absorb nation" agreement, or else their armies would have flipped, too.

Might be a bug in that possibly Rome may have been trying to acquire all of Etrusque's regions and accidentally gave all of it's own land away instead, but that type of bug would likely have surfaced before now and been squashed already. So I think it's more likely that Etrusque has a leader with a high diplomacy skill and got lucky with a very low odds proposal.