Aha! others also seeing this issue.
I just posted about it (sorry, searched forum and didn't see your post 'til now). Here's my thread:
http://www.emaildiscussions.com/showthread.php?t=72975
And I included a video of the problem, on my Google Pixel XL.
https://photos.app.goo.gl/U7GDoay9MKMzNoUH2
It does seem related to the Google Keyboard, but it also seems unique to Fastmail. I can't change keyboards easily on the Pixel and don't really want to because I do love the GBoard in every other app.
Help!
Thanks,
<M>