I got a reply to my support case:
This is a known issue and is going to be fixed in version 10.2.300.15 by ERPS-108625. I have linked this case to the problem record so you will be notified of any changes to the target version but at this point expect it to be available in 10.2.300.15 so upgrading to that version once available will clear the issue.
The work around for this issue is to paste code into the expression editor that contains a period.