Power Up Button returns at Level 38
Back when they first fixed the bug to where we can level up 2 levels past our current level; players at level 37 who maxed them out to level 39 had their power up button disappear. A few repeat posts about the same dilemna were created but no one had proof if it came back at Level 38.
It does come back.
Finally hit Level 38 and it came back. Already maxed out 3 to level 40 and waiting on 3 more to come back from defending .

Answers
I know. Not going to dig through these forums but I was the original one posting about Level 37 Players maxing out a Pokemon to Level 39 only to have their option to power up (button) completely removed.
Every other level below 37 who maxed them out 2 Levels passed their current level only had their option greyed out further saying their trainer level was too low.
Why Level 37 players with Level 39 Pokémon with no grey button was beyond us. We weren’t sure if we would get it back since the update “fixed” the button being removed; but that was for players already at level 38-39 with Level 39 Pokémon , not Level 37 with Level 38.5 Pokémon before the update.
I used this screenshot for that post 2 updates ago. It’s a Level 39 Charizard.
Yup I remember the post (I have the exact same Charizard, so it stuck in the memory).
They claimed they fixed it...but it wasn't fixed. They then removed that update because it had many other bugs on top of the 'broken fix' to maxing pokemon.
However the following update fixed it properly, about 2 weeks ago now.
Congrats on hitting 38 homie :) now the grind for dust begins lol. It's nice having level 40 pokemons. Going to cost a ton of dust powering up everything from 39 to 40, but it's worth it at the end. I love knowing if I have 15 attack/perfect iv, I will be doing max damage possible in a raid and everyone can count on me when I show up to raids. Plus it's nice raiding with other people with maxed out pokemons, just 3 man destroying raid bosses like entei. Plus u and me can 2 man victreebel :)