Does the power up button return after you level up to finish maxing out a Pokémon?
I’m curious cause I’m only level 37 but all my Pokémon that I’ve pushed to Level 39 have the button removed.
My wife is Level 35 and pushed a few to Level 37 and she still has her button greyed out and still says her trainer Level is too low to do another one.
Mine are completely gone and I hope they return so I can further push them to Level 40 soon.

Answers
Updated to the latest version yesterday where you can finally Level up 2 levels past your current level. Then today grinded enough stardust to push some of my previous level 38.5 Pokémon to level 39 today. It wasn’t until about the 5th Pokémon I pushed towards level 39 is when I realized the button disappeared.
Went back to check the other level 39 Pokémon and sure enough they all had the power up button removed.
Hmm, interesting. All I know is that if you're not at least at level 38, you won't be able to power up your Pokémon to level 40. Why the button disappears is beyond me, though. That should not be happening with the latest update, as I was able to power up some of my Pokémon that were stuck in level 39 to level 40. Very weird.
Maybe Niantic changed it in that now, you have to be at level 39 to power up your Pokémon to level 40, so once you hit level 39, the power up button might come back?? Not too sure.
I'm level 39 and the latest update version 0.83.2 (I got it today) enabled me to finally power up my best 15/15/15 Tyranitar to 3670 (fully maxed out). My power up button for him had been removed and I was stuck at 3617, but again, now at 3670. Now its off again, but I know 3670 is the max possible for a Tyranitar.
Have you powered up your charizard before rework? I caught my charmander right before moltres release, evolved it and never powered up b/c it's completely outclassed by moltres/entei. Lvl40 charizard performs a bit worse than lvl30 moltress.
I'm so sad about it, it's my only somehow useful 100% pokemon
UPD: forgot that I caught pinsir recently, I guess it make more sense to power it up
I’ve been powering up it since I got it. It’s not useful now but when Mega Evolutions come , I want my 100% Charizard to be Mega Charizard X, depending how Niantic deals with Mega Evolutions. Also have a Level 39 100% Blastoise that has no use right now but I already invested in it so I don’t have to in the future for his Mega Evolution as well. For Mega Charizard Y, I have a 98% one I caught from a Raid that I’ve yet to invest in but will as we get closer to that generation.
Been trying to do a Maxed out Gen 1/2 starter theme gym and throw my Blastoise in it but it never happens.