Hello,
The Cisco documentation states the below regarding the High and Low options:
Low priority specifies that all prefixes have the same eligibility for protection. High priority specifies that only high-priority prefixes are protected.
You have mentioned above that OSPF treats loopback and /32 prefixes with higher priority, calculating an LFA for these a bit earlier than other prefixes which contradicts the Cisco documentation.
I am a little confused 
Regards,
Kamal