Help, I have no idea on this one

DWC running constant half strength bloom nutes from veg onwards, been going well until last day or two, PH was low and has now been corrected but is this lockout, nutes, heat or something else, some leaf tips curling up, some normal some curling down, majority of tree and roots look fine but just today it has worsened dramatically

It is or at this point only happening to the largest oldest fan leaves

Looks like heat stress to me May be wrong but 85% sure Chill man plant still looks fantastic.

I would say it looks like heat too the twisting was from the ph but you fixed that nothing wrong now she looks great