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.