Nah, just sticky, not uncommon. Put the other piston back in, rebleed, secure the "loose" piston so that it can't move, then when you pump it the sticky one should move. Once it's out, thoroughly clean it and the seal, and the "gutter" in the caliper where the seal goes, then carefully reassemble and see if it still sticks.
It might tbh, and the drawback of Shimano is it's hard for mere mortals to buy individual parts. No bother though, when one of my XT pistons went sticky I bought a brand-new XTR caliper for £15 off ebay complete with pads and swapped 'em 😉