Bubbles on wall

Hi everyone! A little backstory — we bought our house in 2020. I painted the whole house, and several of the walls on the second floor of the house bubbled a ton when painting. I had no issues downstairs. The house was sort of a flip (redone by the previous owner’s father’s construction company). We noticed a lot of short cuts (literally painting over outlet covers) so none of this really surprises me.

I’m finally getting around to fixing it, and I want to do it right. I included some photos of what I’m dealing with. I’ve done so much research on this and I just don’t know why it happened! A few things:

  • I did clean the walls before painting the last time (which was ~November 2020), and I didn’t roll too fast (I deliberately went slow!).

  • I used a ⅜ inch nap roller from Lowe’s (definitely the cheapest, though - does this make a difference?)

  • I bought the “showcase” Sherwin Williams paint.

So my questions:

  1. I’m sanding the walls to start fresh. Is that the right idea?

  2. Why is the paint literally peeling up (see photo)?

  3. How can I try to avoid this now? (I’m going to run a dehumidifier, as I live in CT and it’s humid, plus an air conditioner. I’d really like to paint this summer since I’m a teacher, but I know it’s quite warm…)

  4. Is it possible they used a different type of paint? (I.e., something not acrylic like I bought at the store?)

I consider myself pretty competent; I research a lot and I’m able to DIY things pretty well. This has really tripped me up. Thanks so much for your help!!