Lock button only works when back cover is removed

I've discovered that my phones lock button only works when the plastic back cover is taken off. The cover doesn't obstruct the button's motion in any way. Even if the cover is only partially on (so that it isn't even near the lock button) the problem still exists. I've restrted the device many times and this issue has persisted for about a week now. When the cover is on I can still press the lock button and it has its normal tactile click but serves no function. It wont power on, wake, sleep, or power off the device. When the cover is taken off, the functionality of the button comes back after 3-5 seconds (not instantly surprisingly enough). Very strange issue that I have no clue what could be behind it.