I noticed the MEGA328PB 2017 datasheet (doc # 40001906a) says (incorrectly, I believe) (sections 19.3 & 10.6.1:
Note: To perform a 16-bit write operation, the low byte must be written before the high byte.
For a 16-bit read, the low byte must be read before the high byte.
This doesn't even match their description in prior sentences (When the low byte of a 16-bit register is written by the CPU, the high byte that is currently stored in TEMP and the low byte being written are both copied into the 16-bit register in the same clock cycle)
For the past decade, the various old datasheets always said:
To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read, the low byte must be read before the high byte.
the 2015 mega328pb datasheet gives the high followed by low approach
...looks like someone did some careless editing on the 2017 update??