When using the Message field (for edit) custom field the message appears correctly on the issue transition screen:
.
However when I went back to edit the default value the contents are no longer there.
I get to the default value via:
Admin -> Custom fields -> [custom message field] -> Configure -> Edit default value
I went back and re-entered the default value as follows:

I clicked "Set default". Then immediately clicked "Edit default value" again and the field was blank. Altho on the transition pop-up screen the value is still showing correctly.

I think the app for this custom field is the JIRA Toolkit Plugin. It is installed and 24 of 24 modules enabled. Any help appreciated!
thanks, Sandra