I am having this exact same issue! My Behavior Tree Tasks have their variables resetting when I close UE and launch it again.
UE 5.4.2. I followed the tutorial here: https://www.youtube.com/watch?v=-t3PbGRazKg
I have recreated the Behavior Tree and its Blackboard from scratch, and the exact same thing happens on this behaviour tree.
Moreover, whenever I fix the values (doing nothing else after the launch of the editor) and view the changes in Revision Control, the diff has nothing to do with the broken BTT!
This is the diff that UE detects after fixing the variable values, and please note, neither of these tasks are the ones I changed, and their values were fine:
This is the task (there is a second instance of it in the BT) that always has its Movement Speed set to Idle (instead of what you see here). Changing this value from Idle → Movement Speed resulted in the diff above! 
This is breaking any ability to even follow a tutorial 
I will try next by creating a new Behavior Tree Task to see if the current one is corrupted somehow?