As reported by Navak:

Hiding the loot frame is undesirable when the loot method is set to master looter. Navak hinted that even manually (read: not auto) looting a mob is insufficient for master looting.

I made several attempts to hook events for loot method changed and enable/disable hiding/fast loot functionality based upon these changes, but was unable to come to a complete implementation that covered edge cases where the loot method was changed in the middle of looting a mob.

Introduced in 1204781.

Reverting 1204781 until issue can be resolved.

I will not be reimplementing frame hiding. Closing.