Thank you @dmertz for this crystal clear explanation. This was my first rollover, and I was quite ignorant about reporting mechanics.
Re: IRA custodian -- I do agree now, but this was Vanguard! With its storied history, and blue-chip pedigree! And I wasn't doing anything exotic. Yet, not only did they bungle this seemingly straightforward rollover; they were not helpful at all in trying to resolve the situation. I spent hours on hold trying to reach someone who might point out a solution, before posting here.
Note to self and other novices: scrutinize with eagle eyes the funds transferred into the target plan when you do a rollover. Had I caught the missing 10% then, I could have pursued it with VG and they might have rectified it. Instead I just eyeballed the transfer and thought it "looked about right". Would've saved a lot of time and a modest amount of panic in the run up to tax day.
