Just a guess......but maybe the pH is a bit too low and you are seeing the first signs of lock out/a small bit of lockout? I'm not a fan of that medium for that reason. Peat is seriously acidic....and like coco and salt....varies from batch to batch from my exposure to it. I see some guys rockin it but that was one medium that simply gave me Hell. I see the spots....but I see the green color variations more. Could be ferts/lack of...but if it's out of the zone of absorption...no/very little N will hit it's mark. Can't say for sure. You could try a foliar feed/spray. If they are having a hard time down below they may accept a bit up top. Kinda grabbin at straws here without knowing the pH/etc.
But..it could also be genetics. I have a plant outside that has weird dots I can't remember seeing before....and very slow bud development. I'm not chasing pH/etc.... or worried...so I';m letting it go and figure she will just take far longer to finish/that it's her problem..not something I did. She's the only offender....everyone fed the same/same medium/etc. As they say "Shit Happens".
Good luck!