Why do my corners lift on longer prints?

Nine times out of ten what you're describing is the plate slowly cooling down or drifting in temprature over a long print, not your first layer. A first layer can look perfect and still let go hours later once the part gets tall enough that contraction forces start pulling on the corners. On my machine the thing that finally fixed this was running a brim on anything over a couple hours, plus bumping the bed temp up a bit for long jobs. A brim on the corners gives the shrinking plastic somethig to grab besides itself, and it snaps off clean afterward.

That open spool is prolly doing more damage than you think. I had the exact same pattern a while back, short prints fine, long ones failing partway through, and it turned out to be damp filament making the layers weaker so the corners had nothing holding them togehter. I stuck mine in a food dehydrator at 45 degrees for about 6 hours and the difference was night and day. Since it's only PLA I'd try drying what you have before buying anything. If the fresh-from-the-dryer spool still lifts, then you know it's the machine and not the plastic.

Funny how it always lets go right around the two hour mark, that was my exact pattern years ago. Turned out my print room dropped a few degrees at night when the furnace kicked down, and the draft hit the open front of the machine. Long prints were fine in summer, cursed in winter. Old habit from my ABS days, I just aimed a small box fan away from the printer and shut the vents in that room. PLA hates moving air even more than it hates cold, so have a look at what your house does overnight before you spend a dime.

Mouse ears sorted this out for me. Little round discs under each corner in the slicer, connected to the part. The corners that used to peel up after a few hours stayed planted the whole way through. Costs nothing to try before buying anything, just takes a minute to add them. Also check your plate for scratches or shiny spots, my original sheet got worn smooth in the corners from scraping and that was half my problem too.

Same two hour wall here last winter, tried all the usual stuff and nothing moved the needle. Turned out my original plate was just done. Months of scraping and washing had taken the coating off in the spots where parts always sit, and no amount of soap was bringing it back. I stuck a fresh sheet on, re-did the bed leveling, and long prints just worked again. If yours has been on there a while and you've been scraping parts off regularly, that plate might be quietly worn out. Cheap fix compared to chasing settings for weeks.

Update: tried the mouse ears trick last night and the corners actualy stayed down thru a four hour print, so that alone was worth it. Still suspect the spool though, it does that fizzing sound sometimes when it prints. Is there an easy way to tell if filament is damp without buying a dryer, or is the fizz basicly the answer?