Mine shot loose twice so I drizzled blue Loc-Tite into the screw holes to gum up the threads. So far OK after a couple of range sessions. I snugged them up like you did the first time but it didn't hold. I couldn't believe what flimsy, loose sights were put on the gun.
There was no chance in, well, let's say DC, that the rear sight on my Mini 14 model 5847 was going to come loose. That is until I heated the factory tightened screw with a soldering iron to soften the thread tightener and removed it. The Tech Sight MINI200 I put in its place is great, but I evidently didn't use enough Blue Loctite because it came loose after a few hundred rounds.