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My guess is if you had too much pressure on the corners, you would get a bed that bulges up in the middle not down. Especially if you hit those pegs sticking up from the carriage underneath the heatbed.
I have had both my heatbeds completely off the printer to check them with a metal ruler and they were both lower in the middle than in the corners. Creality makes great printers but I guess their heatbeds are their weakness.
But its a good point you make about too much tightening of the levelling screws, someone else may have that exact problem.