Mark -- I'm resurrecting this thread because the problem came up again in GMF. Apparently there are several groups out there who didn't notice the change in mid 2019 that implemented multiple invite/direct add templates. They expected the previous behavior that auto-populated the invitation with the "active" member notice. As a result, they've been sending invitations for more than a year without any custom text at all.
For those of us (and I suspect it's a substantial majority) who have no need for any multi-template feature, it would really be nice to be able to set a default/active template and have it auto-populate without having to select a lone template from the pick-list time after time.