extreme_one having read the OP of this thread a few times to make sure I've got my head around it (possibly),I think the changes in payback that members have seen since the move to Flarum is much more likely to be because of the change in the way that threads are considered to contribute towards enhanced payback here in the general community.
Since the launch of Payback 2.0 the system for rewarding general contributions to the forum has been like this:-
Lastly, we’ll be looking to reward members who start, and/or take part, in the many great discussions that take place on the community. We’ll be doing this by looking for the most active and popular threads each month, and then filtering through them to remove things like forum games. Everyone who participates in those threads that we feel contributed the most to the community each month will receive Enhanced Payback based on how active they were in those threads, and how popular their contributions were (using a similar kudos approach to that used for Basic Payback)."
So if the way those threads are selected and the number of them used has changed since the move to Flarum as mentioned by @alex_w in his OP,members could find themselves in a situation where content that would have counted towards payback with the old Lithium platform doesn't count towards it now.Hopefully though,once the selection process is refined to bring it more in line with how things worked with Lithium,any differences should get ironed out.
If anyone wants a look at the original thread from 2015 where Alex explained just how Payback 2.0 works,you can find it here:-