Yes I have noticed this happens more in outdoor grows and itโs quite normal and not as concerning as if you had yellowing dying leaves in and indoor set up. Several of my outdoor plants have started with some yellowing a dropping a few leaves also.
I donโt believe WPM would have anything to do with this..
I also realized I was likely underfeeding a bit as you mentioned.. Iโve got some big girls outside and wasnโt used to their nutrient needs. That said I wouldnโt go slamming it with nitrogen - seeing ass your flowering has begun. Maybe up your source of N for one feed then go back to your normal feeding schedule!