Additional notes: Rimpy gave me an error message that a file was missing from advancepowerplus that could not be downloaded from steam no matter how much I tried, so it appears a file went missing somewhere in the update push.

Downloading advancepowerplus from Github made everything work again (as github has not been updated for 1.3 lol)

I have reuploaded the mod on steam. It is really bad if some files can't be downloaded X_x And no idea how to fight that really.

Regarding the loading problem. I had to change mod's ID from "sulusdacor.james76543.Meltup.AdvancedPowerPlus" to "Meltup.AdvancedPowerPlus" in 1.3 due to some visual bugs with it. This would remove the mod from mod list on first load but once you readd it, mod should stay there. Can it be that mod list doesn't get updated for you? Try the following:
find mod list in
Users\%USERNAME\AppData\LocalLow\Ludeon Studios\Rimworld by Ludeon Studios\Config\ModsConfig.xml
find
"sulusdacor.james76543.meltup.advancedpowerplus"
in the text there and change it to
"meltup.advancedpowerplus"

Github version will work with 1.3 but there are a lot of visual bugs with it.

Alright, that worked, thanks!

And if anyone else has this problem: Your save game will throw up the usual "your mods look different" error notification, (even though the list will look exactly the same), just let it load anyway, then save over it. if you load from your save game's mod list, it will revert the changes in modsconfig.xml XD