What happened to the power up button?

You level is ? 37?
If yes you can power up it again after leveling up.

It is whatever the original bug was that removed the power-up button when a pokemon reached level 39. They managed to finally fix that bug, but somehow the fix only applies when players are level 38 or higher. When you reach level 38, the power up button will return. When someone else reaches level 37 two days from now, they will ask this same question.