Yeah, my SPG # is also eleven digits, but I was only a member while there was an SPG program. My Marriott # is 9 digits and I've been a member for over a decade, but do recall people with shorter MR # before.

It seems that MOST Marriott Rewards members have 9 digit numbers -- and the expectation is that those numbers stay. Those people with either conflicting SPG #s or numbers that aren't 9 digits are being migrated.
I am expecting to get an email for my SPG account that I'll have a new Marriott Rewards # for that account .. and then I can choose to combine that with my legacy Marriott account (or take the new number).

Hopefully since many of us are in 'standard' 11 digit SPG and 9 digit Marriott situations it will go OK for the majority of us.. but I think the ones who don't fit that mold might run into problems like the OP. Good luck!