I have I have a text shortcut to enter email addresses. When I enter an email address it appears to be accepted but when I Tab out of the two line I get an error message that there has to be an email address. It may be that I can make it work by backspace deleting and eliminating the highlighting of the email address and then tabbing out. I’m still testing that to see if that works consistently.
I could reproduce the problem with K-9 Mail 5.600. It doesn’t seem to be an issue in the beta version.