$vardef.default as default value

Some time ago I did an upgrade and after that upgrade I had noticed that the default value for some of the fields in Studio are set to $vardef.default.

I didn’t think much about it at the time, just thought it had something to do with the upgrade.
However, now I am seeing these in the database as the value in many of my custom fields.

I presume I can just remove this from the default field value but I was wondering why this might have happened. Also presuming I can clean up the records in the database that are affected.

Any thoughts?