deficient rust spots can't figure it out

Closed room
air-con
bottled co2
High lights LEDs

room temp 28 degrees celsius
room relative humidity 50%

Flood and drain x trays with no medium, roots just grow on the tray ( KLX WAY ) if ur familiar with his growing style

Feed is
PH 5.5
Canna aqua a + b 3ml per litre
Canna Cal mag 1ml per litre
canna PK 1 month into flower

I still pull down good numbers, but after many grows with the same strain I can't figure out what is causing the rust spotting.

try running it at 6.2 next grow cycle.

did you ever figure out what was causing this?