I have considered it, and I'm looking at alternatives.
When I first started to suspect the flash code (only two, three weeks ago!), I started digging, and I found (very conflicting!) information that you can't use "certain" (which ones depends on the site) GPIOs and/or (!?) ADCs at the same time you're using the flash.
I have another question about that here, but nothing conclusive yet.
BUT, because it works in read_button().. That don't seem plausible. Same program, just another task. And core, but it didn't work before I moved it to core1 either, so..