What happens if an interrupt callback execution is interrupted by the same interrupt?
The HAL timer interrupt handler does not do anything special, it checks all possible pending interrupts, clears them and runs the callback. So it is not re-entrant. If your callback takes longer to run than the timer period, the interrupt has to wait until callback and handler are finished to run again, so during this time the interrups are missed and it will only run as many interrupts per second it can.