Hi Lorenzo,
there were many bugs in the generic CBCH/SMS-CB support of both OsmoBTS and OsmoBSC. It was working once but has bit-rot over time. Due to a lack of related regression tests, this went unnoticed so far.
The fixes are now all merged, so if you build current master of osmo-bts and osmo-trx, you should get working CBCH support.
I've also put an example at https://osmocom.org/projects/cellular-infrastructure/wiki/Cell_Broadcast#How...
Please let me know if it works for you.