[Evolution] Why does Thunderbird work and Evolution fail?

[Anonymous Japhering <japhering gmail com>]

Thunderbird  78.13.0 (64-bit)
Evolution 3.40.5 (flatpak gite4cdc88)
Kernel: 5.4.0-84-generic x86_64 bits: 64
Desktop: MATE 1.24.0
Distro: Linux Mint 20.2 Uma

So I currently have 8 Gmail/G-Suite accounts that use imap and oauth2.
As of my upgrade to LM 20.x, Evolution fails to authenticate into the gmail
environment.

Every time Evolution attempts to connect via imap, I get the login 
dialog, followed
by the request to allow/deny Evolution access to my account. I provide 
the required
information and nothing happens.  No list of emails, no previews, no 
indication of email being available.

At the next time interval or the next time I click Send/Receive, the 
same things happens.

The only clue seems to be that on the 2nd and following attempts the 
login popup says "Oauth2 secret not found".  I haven't cleared any 
caches intentionally, maybe accidentally.

Yet,  same set up, same machine Thunderbird is running flawlessly on the 
same 8 accounts.  Checking the Google account pages, shows for every 
account that both
Evolution and Thunderbird  are authorized.

So why does Evolution fail when Thunderbird succeeds?

What did I screw up?