At LHR and IAD, past level 80 follows the linear function while at BKK the quadratic function is used.
The XP requirements seem to not be calculated automatically and are likely hardcoded into the game. Aside from the above being an evidence for this, another colloboration is the incorrect BRI XP required on level 50 which was fixed about 9 months ago. Furthermore, the pre-2.0.1 XP requirements did not seem to follow any sort of calculation, and it was after they adjusted the XP with the help of a calculation did it seem like it was calculated by the game itself