Since I discovered this error, I have done it without IF transfer, but filled out a Simbrief beforehand, then generated it and then logged in the Simbrief account from there using the IF Flightplan Tools app, then copied the FPL and data and boom, the results came out completely different than those from the new update.
With Simbrief, other flight levels were also displayed, although the correct ones, with IF transmission through Simbrief showed me that completely different flight levels.
Example at KIAH before the final 8000ft, :O at Simbrief it says 3000ft.
I had this problem with pax and cargo as well but it turns out that the explanation (and solution) is relatively simple:
for pax, you have to create your own airframe first as the airframes in IF are different from SimBrief. if the number of passengers still exceeds the number shown in SimBrief after loading the FPL from SimBrief, then it must be the number of cabin crew (e.g. if SimBrief shows 250 pax but after loading it, it shows 257 pax in weight & balance, then the 7 passengers must be the cabin crew + pilot)
for cargo, well, I think the explanation lies in the thread you quoted