What does this issue looks like?

Hey there folks ... I have hard time determining what kind of an issue I got. I flushed and then fed again my plants 'cause I got some issues with Nitrogen 8 days ago since 6 days all went back to normal and 3 days ago I started seeing different kind of deformation on the leaves which I think is a Magnesium deficiency or possibly an over watering sign? I also don't exclude some calcium deficiency perhaps, but I need some guidance.

Pictures:

These are all of the 5 leaves that have such a signs on this particular plant of mine. I don't see the issue spreading for now but it is increasing to damage the leaves. I have 2 other plants in the same stage of flowering(55 day), having 4 and 1 leaves with the same issue. I have another plant just entering flowering (11 days) and another 4 clones which are day 1 flowering which have no such signs but had been treated with flush & changed in the feeding program as well. All 3 plants with the damaged leaves are Sativa dominant so flowering about 90 - 100 days is expected.

from the sound and looks of things, it appears that your medium could be wet longer than the plants liking.

you already answered your question .. there in flower and when there getting further along they will deplete the nitrogen from the leaves to try and keep up the the high magnesium and phosphorous that your giving them.. if its a sativa dom then start by adding some more nitrogen in with your nute schedule.. they will always do this during flower..if you look at every late flowering plant the fan leaves are always yellowing out..

I am also thinking about over watering ... Since the flush the soil surface has never had a chance to dry out ... I left them yesterday without watering (I water each 24 hours ... It's just my climate) and they don't seem to drop down and need water for now, so my best guess is over watering ... This by the way happens only on lower to medium leaves but first oldest ones.

Bass1014, they had some intermediate to serious Nitrogen toxicity 'cause of the old fertilizer, I forgot to buy a special one for blooming so it wasn't all smooth during flower now, but ehh, my first ever grow. So the Nitrogen should be fine now they stopped showing signs of Nitrogen toxicity. But none the less, experimenting will assist in solving the issue, I just hope my plants survive this one yet again.

EDIT: I think I know what is the real cause ... Nute burn, I noticed that these three particular plant containers flushed for like 2 - 3 hours each, fully ... So there may yet be nutrients from the old nutrients in the solution and now they are just burning parts of the plants ...

The leaves are at the end of their life cycle after using up their reserves of energy, as Bass 1014 stated. At that time yellowing is normal, so don't freak out. Peace

Hmm, but I still have about 50 days to go ... lol

Inexpensive soil NPK test kits take the guess work out of what she needs or has too much. IMO it looks like overwatering n nute burn. Less is always more. Until I started using a test kit once a week what I thought was deficency was really surplus burning off.