Guys quick update - issue resolved!
Culprit: Evo stick adhesive
Resolution: white spirit the insides of the case, cleaned it with iso propyl + boiling hot water, I used generic PVA glue to line the case with thick board to block light leak, used duct tape to eliminate light leak completely, and used rustoleum white matte spray paint instead, let it dry for 72 hours with exhaust fan running max speed and a heater inside to dry/cure quicker
Plants have been in the case for 48 hours now, no drooping, visible growth and no dodgy colours n shit.
Also moved over to chemical pH strips, thanks for all the help!
Avoid Evo stick adhesive, will definitely make your plants trip!