Weird issue with html emails

One of the sites I have Formidable on is having a bizarre issue where the html emails are seemingly randomly going back and forth between displaying properly and coming through with the html code visible.

It seems to work fine every time I test it myself, but when normal users submit the form, it’s a toss-up as to whether the email we get will show properly or display the html code.

We’ve got two forms, both using just the basic [default-message] for the message, and the issue is happening with both of them. I’ve got Formidable on lots of other sites, mostly on the same server as this one, and haven’t run into this issue with any of them, it seems to be exclusive to this site. I’ve had to switch it to plain text emails for now, but would like to get it straightened out so they can be formatted like they should be. Any idea what might be up with this?

https://ww.wp.xz.cn/plugins/formidable/