Unable to map longtext field in Record Type relationship

Hi,

I have a field 'allocatedTo' in the database table with longtext type, which is translates to Extra Long Text type in Appian Record.

While trying to edit a relationship with another record having 'username' field as Text type, 'allocatedTo' does not show up in the dropdown.

Strangely, it worked fine first time, now shows error in the relationship.

What could be the issue ?

Thanks.