iPad mini 2 won't manually lock after iOS 12.5.8 update

I've got an old iPad mini 2 that won't lock. This only became an issue after I updated to the latest supported software version 12.5.8. Pressing the power button won't lock the iPad, however holding it down will turn off the iPad as usual. Additionally, Assistive Touch does not lock the iPad, so I'm assuming it's a software issue. One thing to note is that the iPad will lock using the auto lock feature after some time. Any other solution would be great. Thanks!