WORKAROUND
If this happens to you, here is what I did to resolve the issue.
Cause is not fully known, but it appears to be related to marking a Pending donation as Completed within the donation record itself (WP -> Donations Tab -> Donations tab -> select donation).
Aside: Why the main tab in WP is “Donations” and not “GIVE” is beyond me.
Download all transactions using Tools -> Export
Find the donation that is not appearing in your donations and get the Donation id
Now go back to donations and open any other donation record.
The id of the donation is the last number in the URL.
Replace that number with the missing donation id.
The donation status will be Pending.
Set the status to Processing and save
Set the status to Complete and save.
This may make the donation appear as doubled on the website form.
If this happens, go to Tools, then Data.
Select “Recalculate Revenue Amount and Donation Counts for All Forms” and Submit.
Clear your cache and all should be good again.