For starters, Pex is it's on thing, with crimp fittings. As for it spinning, you need 2 wrenches to take the cap off, one to hold the pipe, the other to spin the cap. All the fittings are plastic, and won't stand a lot of pressure from twisting or bending. Look in the cabinet above the drain line, and maybe you can see the crimp connection. Borrow or rent a crimp tool, Pex 1/2 inch, and maybe re-crimp that connection.
By the way, shark bite connectors seal with an O ring, that is why they will spin.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT