This is a polyester shirt from the Uniqlo airism range.
I like the shirt, but recently it got these spots on it? Is this from sweat?
This picture is after washing with lots of detergent.
Is there a way to get it out?
It seems to disappear after a while, but comes back after washing.
The shirt also smells ok, but after wearing it for a short while it starts smelling.
Okay yeah, that would be my guess as to what that is then. Honestly, not sure why it goes away for you, as my elastane/spandex blend clothes have just been relegated to the “house clothes” category now. I could be wrong but it looks just like it from what I can see.
Polyester does hold on to scents like crazy in my experience. If it’s not washing out try some isopropyl alcohol in a spray bottle and giving it a spritz. Different textiles all have different properties, but I almost always recommend natural fibers for most purposes.