Hey guys, I might be able to clarify a few things. Disclaimer, the information I'm about to give you have not been given to me by game employees but purely my opinion.
In my opinion, this maintenance is only activated on devices which have accounts on one of the servers scheduled to be merged(17-33). This sounds like the most plausible reason for maintenance to be conducted only for some players. I might be wrong, but I only have one device with an account on servers 20 and 33, and It is the only one that cannot log into the game even on my main in server 100. My PC however, can log into server 1037 no problem(no accounts on other servers)