Plaintext emails do not display

Summary

When I disable sending HTML emails, the emails I get are blank.

Steps to reproduce

Enable "disable HTML mails" in Admin -> Settings -> Email -> "Enable HTML Mails"

Expected behavior

To see the plaintext output

Actual behavior

No output (looking at the email source shows it, some mixup with mime encoding I'm guessing)

I enabled this on 8.15.3 EE and while I get the emails, I have to look at the source to see the actual content I expect to see. I checked with my mail client (MailMate) and with the Gmail client on iOS and the Apple Mail client on iOS.

This was implemented in issue #24880 (closed) but I'm guessing it wasn't tested?