On a Nexus 5 (running OTA-21) timers running when the phone locks resume "on-time" when the phone is unlocked. For example, a timer with an interval of
one hour triggered at 9:00 will trigger again at 10:00 even if the phone is locked between 9:15 and 9:45. It will trigger again at 11:00 and so on.

On a Pixel 3a (also running OTA-21), however, timers running when the phone locks are delayed by the time the phone is locked. In the example above, the timer would trigger at 10:30 rather than 10:00, again at 11:30 and so on.

The above distinction holds whether or not the app is suspended when the phone is locked.

Does anyone know why this discrepancy exists?

I would think the behavior ought to be the same across all devices and that the behavior on the Nexus 5 is to be preferred.