I think this is a bug.
Actually not. We do this because adding highlights is (currently) only possible on editable fields, and this is a valuable use case for the email integration.
Okay makes sense, but its an integration, it’s meant to show the data from the email. Having the data be editable means it can fall out of sync. And pressing the sync button doesn’t fix it.
Ideally it would be possible to create highlights on read-only field then.
For now I will create a read-only formula mirroring the message, just so people don’t accidentally edit.
I see its actually not possible to make a formula create a rich text field. Just the snippet. However it is possible with a lookup to a relation. Just a note for anyone else trying to get around this… Could to a auto-link self relation based on public id, then pull the rich text field with a look up. but thats a bit messy for just a non-editable field.