I can't say for certain about AR/VR screens in particular, but we know some things about doing extended "close work" with your eyes: https://www.reviewofoptometry.com/article/when-stress-strain...
Nearpoint stress basically can render your eyes permanently damaged. It's definitely to be avoided.
Existing VR displays have your eyes fixed focused either several meters out, or at the horizon, so the kind of eye strain your talking about is not an issue.
MagicLeap supposedly has variable distance focus working.
Do we know the (eye) health effects, yet, of looking at AR/VR screens an inch from your eyes for 6+ hours a day?