I have an update on this issue.
In my ISR I have interrupts created by timer 1 and timer 3. Timer 1 is used as a one-shot timer and timer 3 is used as the low frequency 1Hz generator.

Timer 1 and timer 3 are identical peripherals, so I have simply renamed timer 1 registers to timer 3 registers and vice versa, and now the code works well on 26k22 as well as 25k22, with timer 1 running the 1 second epoch code.

The only difference between timer 1 and timer 3 is that they use different peripheral interrupt enable registers, maybe there is an issue with that ?

Chris