Here are the relevant transactions all in 2018:
3/7 Bought Total International Stock Market Index Admiral Fund (TISM)
4/17 Bought TISM
5/17 Bought TISM
6/7 Bought TISM
6/19 Exchanged all shares of TISM for FTSE EX-US (FTSE)
6/27 Exchanged all shares of FTSE for a combination of Developed Market Index Fund + Emerging Market Index Fund (Dev+EM)
8/8 Bought some more shares of TISM
8/20 Exchanged all shares of TISM and all shares of Dev+EM for FTSE
9/21 Bought TISM
9/28 Exchanged all shares of TISM for FTSE
10/9 Exchanged all shares of FTSE for Dev+EM
10/15 Exchanged all shares of Dev+EM for a combination of Europe Stock Index + Pacific Stock Index + Emerging Markets Select (Eur+Pac+EMS)
Vanguard is convinced that SOME (and only some) of the FTSE shares purchased on 8/20 and sold on 10/9 are wash sales because of the FTSE shares purchased on 9/28 (which were also sold 10/9). Specifically, the shares they do not like are the ones exchanged from TISM and EM to FTSE on 8/20. The ones exchanged from Dev are apparently fine. I don't recall the order I put the transactions in, but perhaps that has something to do with it.
I should note that this is purely a theoretical exercise. I have been credited all the losses I deserve, because what was taken back by Vanguard for 10/9 was subsequently harvested without issue on 10/15. But if my understanding of a wash sale is wrong, I'd like to know about it.
I'll post some screenshots that may help someone figure out what's going on.

And with more detail for the FTSE fund.

So whose error is it? Vanguard's or mine?