Yes, you're definitely hitting boost cut!
The stock ecu's max out their small amount of "adaption" at 12 psi (anything over that and you'll be running lean) and yes will hit boost cut around 13psi unless properly de-restricted and/or tuned. Your safest option right now is to bring boost back down to 12 psi in 4th/5th gear, return the stock bov/bpv or add a wrx equivalent (resurculating) and unplug the battery for an ecu reset (there's a good how-to on here somewhere).
My best advice is to be safe, rather than sorry. Keep the psi low (12psi) for good afr's and save yourself for a decent tune to maximise gains!