Can't send emails from shared mailbox

And what happens if you click From > Other email address > select the shared mailbox from the GAL?

Then I can send from the shared mailbox (just like I can if I manually enter the shared mailbox email address in From field), but it does not appear in the From entries and does not default to that mailbox as the sender the next time.

Until a couple of weeks ago, everything was working just fine, as it has for several years.  The shared mailbox appeared in the From list and was the default sender when creating new emails and answering emails sent to that account.  (I have a total of five Exchange Online accounts set up.)  Then suddenly it stopped working -- on two Windows laptops. 

From looking at the releases at https://learn.microsoft.com/en-us/officeupdates/current-channel it looks like these issues started happening with version 2501 on January 30.  Just to re-iterate, everything works as before on the Outlook web app and iPhone and iPad apps.  This has me thinking that there's an issue with Outlook desktop starting with 2501.

If you want the entry always visible under From, you will need to add it as separate account. 

I believe I originally set this up using this method.  Microsoft support suggested creating a new profile, but I hesitate to try that because I'd have to resync thousands of emails, set up all my email signatures again, and since we're in my busy season I'm working very long days and working on fumes.

Detailed steps here: https://www.michev.info/blog/post/3567/how-to-add-a-shared-mailbox-as-additional-account-in-outlook-2022-version 

I followed those steps to add the shared mailbox as an account and now when I expand the email tree on the left hand pane I get a "Microsoft Outlook cannot expand the folder" error.

Any ideas on what to try next?  I'm just about ready to ditch Microsoft/Exchange Online over issues like these.