At 90 rounds, either last of correct cleaning and lubing to begin with  (rifle is fouling choking out),

Or, something came loose and is leaking, such as the key to carrier, or even gas block to the barrel.

Forget the bore snake, since you really should be cleaning the barrel chamber with a chamber brush and CLP by hand to start with to get all the fouling out of the chamber, then the  barrel bore cleaned afterwards with something like Sweets copper solvent with brushes and patches.  This goes even more so if you are shooting steel case ammo, or just ammo that the bullets have been sealed to the cases.

As for the key to carrier leak, could be a bolt snapped or key metal was not staked into the sides of the key bolt spines to hold it in place, where as the gas block allen bolts should have been loctite in place during the build.
Note, on the gas block allen bolts, if you need to remove them at a later date, use a torch to heat the bolts enough to break down the loctite bond before you spin the allen bolts back out.  Failure to do so, will end up with just the head sockets of the allen bolts stripped out instead.


Lastly, on a new build, around the 300 round mark, not a bad idea to pull the FCG out of the lower, to give it and its channels/pins a good cleaning.  The inner channels and pins are parkerized, and about that mark, most of the parkerizing has worn off the bearing areas.  I bring this up, since with the FCG removed, you can check for any other foreign debris that may be hiding in the lower receiver cavities as well.