The Control A (WDT.CTRLA) register is synchronized when written, due to the asynchronicity between the WDT clock domain and the peripheral clock domain. The Synchronization Busy (SYNCBUSY) flag in the STATUS (WDT.STATUS) register indicates if there is an ongoing synchronization.
Writing to WDT.CTRLA while SYNCBUSY = 1 is not allowed.
The WDR instruction will need two to three cycles of the WDT
clock to be synchronized.