According to TCRF, there was supposed to be a check in the Arcade mazes to have levels 20+ have 0 seconds of power pellet time, however, this gets overwritten by a later check capping the timer at 2 seconds instead, which was only intended for the non-arcade sets.
Furthermore, in the 16-bit versions, rather than capping at level 19, level 20 resets back to level 1's timer, and goes back down the list up until the final level.
By @Flicky1211:
As for Tengen Ms. Pac-Man, these are the power pellet timers (translated best I could via RussianManSMWC's disassembly).
1: 6 sec 2: 5 sec 3: 4 sec 4: 3 sec 5: 2 sec 6: 5 sec 7: 2 sec 8: 1.75 sec 9: 1.5 sec 10: 4 sec 11: 2 sec 12: 1.75 sec 13: 1.5 sec 14: 2 sec 15: 1.75 sec 16: 1.5 sec 17: 0 sec 18: 1.5 sec 19+: 2 sec
According to TCRF, there was supposed to be a check in the Arcade mazes to have levels 20+ have 0 seconds of power pellet time, however, this gets overwritten by a later check capping the timer at 2 seconds instead, which was only intended for the non-arcade sets.
Furthermore, in the 16-bit versions, rather than capping at level 19, level 20 resets back to level 1's timer, and goes back down the list up until the final level.