Intermittent fatigue, light-headedness, bp drops to 90/60 when standing. ecg, exercise stress echo, holter all normal. vegan diet from 2019. could ferritin level drop from 120ng/ml in 2015 to 40ng/ml in 2022 cause this?

No. An isolated drop in ferritin to a low normal range of 40 should not cause those symptoms. It would be helpful to know where your blood pressure started when lying or sitting. Orthostatic intolerance can present with farigue and a drop in blood pressure when standing. I would book an appointment with your PCP to discuss the possibility of this condition further.

Not ferritin level per se, but you may be anemic form iron and B12 deficiency. A vegan diet may not have sufficient amount of many nutrients, iron, B12 and vitamin D being some. It would be prudent to see your primary care provider for an assessment. Drink enough water so that your urine is mostly colorless, it will prevent symptoms from dehydration.