@88lance With your theory about altered blood flow, I wanted to share with you a source that could support this if you have a condition that reverses blood flow know as "subclavian steal" and it can cause visual disturbances. This would be possible if you have a problem like thoracic outlet syndrome that compresses blood flow and it can be adversely affected by poor posture. I have thoracic outlet syndrome which is diagnosed by confirming altered blood flow to the brain and arms by the position of arms and head (turning head) with Doppler studies and the doctor listening for the pulse to diminish with changes in posture. Have your specialists considered thoracic outlet syndrome as a possibility? Many doctors miss TOS and it is poorly understood and not given much time in med schools. A neurologist or thoracic vascular surgeon would be possible specialists for this condition.

I refer to this paragraph below:

Source:
Vestibular impairment and its association to the neck and TMJ
Posted on July 22, 2018 by Kjetil Larsen

and found at this link:
https://mskneurology.com/vestibular-impairment-and-its-association-to-the-neck-and-tmj/
"Subclavian steal symptoms presents secondary to arterial insufficiency, created by a retrograde flow that “steals” blood from the brain circulation, more specifically from the basilar artery via the vertebral artery. Classically it presents with neurological symptoms from the posterior brain and cerebellum [4,6]. Decreased flow over the basilar artery gives rise to symptoms like lightheadedness, ataxia, vertigo, dizziness, confusion, headache, nystagmus, hearing loss, presyncope and syncope, visual disturbances, focal seizures, and in extremely rare cases, death [6–10]. However the vast majority of patients are asymptomatic and rarely require any intervention [3,5,11]. – Alcocer et al., 2013"

Here is some more information about TOS
https://mskneurology.com/how-truly-treat-thoracic-outlet-syndrome/
https://www.mayoclinic.org/diseases-conditions/thoracic-outlet-syndrome/symptoms-causes/syc-20353988
Have your doctors mentioned TOS? Have you heard of thoracic outlet syndrome before?