The simplest answer is that you're starting to have the very slightest amount of age-related focus degradation, that it's only manifesting when the room is very dark, and that it aligned with the new phones by coincidence.

That said, older phones had a higher minimum brightness and more mixed-in blue light. Your pupils will contract more when looking at a brighter, bluer screen and smaller pupils focus more easily. Also, being able to see the black part of the screen can make it easier to focus because you can see how far away it is.

Normal ergonomic issues like this are probably the cause. Can you get hold of an older model phone and relax with it for a few weeks to check whether the problems are really with the new devices?