I'm gonna go with mag deficiency, and it's been at it for some time. I had something similar.. but not as advanced. Necrosis may start on those leaves that are taking the biggest hit (heavy interveinal yellowing) Add epsom to you're watering. 1tsp per gal.
This pic here shows her suffering from mag deficiency (Close to yours) with added potassium deficiency (Yellowing on the outer edges of the leaf blades)
Other issues she went thru.. over watering (Surprise Surprise!) & low humidity.
At least that's how it was explained to me (Somewhat

) lol Hope all the info that gets rounded up here will help.
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