Im not sure why my veins disappeared recently either.
I've theorized a few possibilities.
One being that muscle atrophy has left me with excess skin which is now hiding my veins.
The other being some lack in nitrous oxide.
Third is that I'm lacking something that is crucial to vitality. Meaning that I associate veins showing with strong healthy blood flow/energy.
But I'll disregard the third theory because it seems you've been getting stronger and I'd imagine that to be impossible if you lacked healthy blood flow and energy.
so could be theory 1 or 2.
I'm also thinking mitochondria but idk.