ich hab mal im daten blatt gesucht
Zu TCNT0 steht folgendens wo wahrscheinlich auch das Problem liegt:

The Timer/Counter Register gives direct access, both for read and write operations, to the
Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the compare
match on the following timer clock. Modifying the counter (TCNT0) while the counter is running,
introduces a risk of missing a compare match between TCNT0 and the OCR0 Register.

Also setzt TCNT0 = 0; bevor du den Timer startest

mfG _Martinius_