My guess its the PH5 that's slowing things down or halting the denitrification. Just briefly googling low ph effects i got this https://www.climate-policy-watcher.o...tion-rate.html. From that article ph below 6 and above 8 depresses the rate of denitrification.Though this study is from water waste systems.
When my test system ph crashed i still had denitrification but it was from established baskets grown at ph 7. The readings i took where at ph 4.3 but i quickly water changed to get back 7. Yours on the other hand is at a constant ph5.