Programming the hardware timers - advanced level

[fungus]

But in your non-working sequence it isn't yet in CTC mode (you update OCR0A and then change to CTC mode) so somehow it loses that update.

My theory is that it's because the register is double-buffered in PWM mode but not in CTC mode.

The value is lost if you write it while the timer is in PWM mode then switch to CTC mode before it gets copied to the real register.