That's no bug, it's a feature!.

Marian and Imperial MICs are in their complex, there is no way to uprgrade a polybian MIC to become a marian MIC. To prevent that players can train both, marian and polybian units at the same we have script that damages the old MIC every turn after the reforms.

At some point we planned to have script that additionally places a marian MIC in all italain provinces after the reforms but don't know if this wa ever realised or dropped.