Guarantee it's because you let thinset build up around the spacer which prevents it from breaking low enough. Cut this one out manually. Use a Dremel if a knife won't work. In the futur, clean off the spacers with your spatula before they dry and this won't happen .

This reminds me of the bike path along the beach with a wall in El Porto/Manhattan beach, ca

I actually wouldn't be shocked if it didn't fall off... Don't get me wrong it absolutely needs to be torn out and redone, but some of these shitty tile jobs I've torn out that were there for 20yrs are complete trash and still held up. I'm talking tiles with barely a dab of joint compound on them. Honestly the grout would likely keep the whole thing up for a decent while lol. But still, definitely want to fix it to avoid the risk