Is there anything else than the MERGE statement in your transaction, like a DDL statement? For what it's worth, I ran into a similar issue, though not on RCSI but with a HOLDLOCK hint, which forced serializable isolation.

How MERGE on two different rows can still deadlock you

I recently ran into a curious deadlock issue. I have a process that performs a lot of updates in a "state" table using multiple, concurrent connections. The business logic in the application guarantee...

Also: Does your table have a single clustered index, or are there non-clustered indexes that are affected by the MERGE?