Index Rebuild with WAIT_AT_LOW_PRIORITY still blocks

I'm running...

ALTER INDEX PK_MyTable 
    ON fct.MyTable 
    REBUILD PARTITION = 261 
    WITH (
        ONLINE = ON (
            WAIT_AT_LOW_PRIORITY (
                MAX_DURATION = 30 minutes, 
                ABORT_AFTER_WAIT = SELF
            )
        ), 
        RESUMABLE = ON, 
        MAX_DURATION = 355 
    )
;

...and I expect it **not** to cause any blocking, but it does. It is asking for a LCK_M_SCH_M lock. My statement is blocked by an INSERT INTO statement, which is what I expect; but it is also blocking many other statements that are asking for a LCK_M_SCH_S lock, and I was not expecting that. At the same time there is a BACKUP running of a database that has FILESTREAM. Could this be the reason? What else could cause this blocking? Have I completely misunderstood WAIT_AT_LOW_PRIORITY? RCSI is on.