[Evolution] Evo 3.12.11: mail from gmail not shown in folder?

[Paul Smith <paul mad-scientist net>]

I'm using Evolution 3.12.11 (on Ubuntu GNOME 15.04).  I used Gnome's
Online Accounts to add two different GMail accounts plus a traditional
IMAP account at my ISP.  All are using IMAPX types in Evo.

Up until today, all was fine.  However, suddenly most of my previous
email in the two gmail accounts is not being shown in the Inbox folder.
If I look at the properties of the folder in Evo it says there are 717
emails in the folder, but the total shown in the folder is only 31
messages.  For the other gmail account it shows only 1 message Inbox,
while the "Total messages" in the properties says 27.  The traditional
IMAP account works fine.

The emails are really there; if I use my browser to go to GMail I can
see them all.  I've triple-checked that there's no search active.  I've
restarted Evo a few times.  I've unsubscribed to the folder and
re-subscribed, and tried "Refresh".  It always gives me the same 31
messages... and these are not even the newest, or oldest... they seem to
be a random assortment of email.  The "Show" drop-down says "All
Messages" and changing this doesn't help.

The really bizarre part is that I have two different systems (one at
work and one at home) both running the same version of Evo and they both
have the same behavior, with the same GMail accounts!  But, I did not
backup/restore them from each other: they were configured separately by
hand, from scratch.

I do seem to be receiving new email and it shows up in the folder.

Any ideas?