you have a couple of options to choose from and considering you have opened the iPad Mini already to replace your part;
- did you alter the two lens assembly magnets in anyway? (located top right and bottom right on device, usually attached to factory lens assembly)
- make sure they have kapton tape covering them (or the original tape that covers said magnets)
- go to settings: tweak the lock screen options, you may have to just swallow the bullet and turn it to off, but when you put a magnetized iPad cover on, it will not lock as it should.
I've a bit that power button flex replacement parts tend to be lackluster quality as far as Apple products go.
good luck,
guy who works on iPads alot.