behaviours: A read-only Version field can be emptied using delete key

We use Jira version 6.2.5 and the Script runner addon version 3.0-beta-9.

Using behaviours, we have made a field read-only (custom field of type Version Picker) for everyone apart from a select group who are members of one particular JIRA group.This seems to work fine and we have been using this for a while. But every now and then we see that a user has managed to empty this specific custom field. This has been a mystery - but now a user has shown me the simple way of removing a value from this read-only field.

How to reproduce:
Open an issue, go to the edit field, locate the read-only field, click on the displayed name of the version so that it is selected (the grey block becomes coloured in) and hit the delete key on the keyboard.

Is there any chance of a fix?
Any help would be appreciated.

(I have just installed Script runner version 3.0.5 but the problem is the same in this version)