"So the rebuild didn't fix it then."

Are you saying so because DBCC PAGE of the rebuilt index points to a different ObjectId? Note that CHECKDB after index rebuild doesn't report...

Any insight is appreciated.

Also, how does a clustered index rebuild work? Is it immune to IAM chain corruptions? If yes, I still don't get why CHECKDB would return repair_allow_data_loss...

CheckDB WITH NO_INFOMSGS on the corrupt DB says this:

Msg 8947, Level 16, State 1, Line 1

Table error: Multiple IAM pages for object ID 318624178, index ID 1, partition ID 72057594663600128,...