This is the Edit form. Both fields already had values that were set in the NewForm but that the user shouldn’t then be able to change. I tried a 1000ms delay with no change.
Would a login to have a look be any use to you?
The field placeholder shows with the hashing that’s normally there for a read only field but no actual field. Screenshots etc. below.
On another list on the site the read only rule works as expected. I seem to have a couple of lists where it just doesn’t. The problem lists were created from the Calendar list template but I’ve hidden all the extra fields like: all day event and recurrence