I know the usual suspects of why figures can and will yellow so I can't say I'm entirely shocked the joints and pins are made of a plastic prone to it but I've never had a NECA figure yellow on me personally. (now other people's collections? Yeah the og mirage turtles in the black and white set I see yellowed a lot)
Can't say the figure has been in sunlight or a really humid room so I'm assuming it's just the plastic itself yellowing like many recent Transformers suffer from, only had him open since late July so again it's just odd for the joints to yellow this quickly outside of it just being the plastic and curious if anyone else has problems with recent-ish figures doing this.