According to the datasheet, the value of the high byte of the counter for 16-bit timers (such as timer 1) is only updated when the low byte is being read/written (page 126 of the datasheet):
The TCNT1H Register can only be indirectly accessed by the CPU. When the CPU does an access to the TCNT1H I/O location, the CPU accesses the high byte temporary register (TEMP). The temporary register is updated with the TCNT1H value when the TCNT1L is read, and TCNT1H is updated with the temporary register value when TCNT1L is written.
Currently, writing to TCNT1H updates the counter immediately.
According to the datasheet, the value of the high byte of the counter for 16-bit timers (such as timer 1) is only updated when the low byte is being read/written (page 126 of the datasheet):
Currently, writing to TCNT1H updates the counter immediately.