I was shooting this weekend, and noticed my POI wasn’t changing vertically when I was zeroing. Screw was loose, and when I tightened it, I could make adjustments. Then within 5 shots it was loose again. I repeated this experience for 4 or 5 iterations, got the rifle zeroed, which held. But, the elevation knob screw is still loose and loosens up upon firing.

Was thinking of loc-tite and seeing if that works, but should I be replacing something? Did I break a clip in there or something?

A screwdriver is needed to turn the split head screw tighter (clockwise) until the elevation clicks become more pronounced. You have to apply inward pressure when doing so

You can modify an old screwdriver into a spanner type screwdriver, but I have used regular screwdrivers without issues.

Is it marked something other than WCE?
Did you purchase the M1A new?

If the answers are no, then it's probably a bad Chinese part and you'll need to replace it.

Springfield Loaded purchased brand new about 3 years back

You'll have to disassemble and inspect the threads to really see what's going on.
SA has that lifetime warranty though.

Your knob is fine. This is the fix required to increase tension on the elevation knob. The split screw mentioned is in the center of the windage knob (on the right side). Give it one (1) half turn at a time. You will feel a slight "click" as it completes a half-turn.

Yep.  Elevation knob screw must be tight.  But tension is set by the screw on teh windage knob.