I'm not sure whether it is intentional or not, but after I have unlocked all summons, completed the main story and now doing side quests I noticed I didn't have my summons anymore and I had to walk up to Mr slave, jesus, Tuong Lu Kim and Mr. Hankey and speak to them again and I get an achievement that says "Summon unlocked!" just like after completing the original quests to unlock them the first time.

I don't get it. Are you meant to go back to each person daily, just to be able to summon them again or is this a bug?

It's pretty annoying.

Yeah that's basically how the summons work... unfortunately they don't reset automatically. Those summons are insanely OP and only good for normal fights any way... there's really no reason to even use them unless your just curious as to how they look.

 

But yes, if you used their summon you have to walk back and talk to each person with each passing day if you intend to use them again.

Mr. Hankey's summon is best in life.