Do't know if this suggestion helps because in my case, I was patching a
hole that I was then going to cover with other flooring, but I had that
situation where about half a tile broke off and disappeared (long
before I lived here).

I pasted over the hole with something called "Patch-it" (I think that's
the name). It was in the same section as the spackle. I was a really
stinky rubbery spackle-type stuff that dries in an easily removable
fashion later on. It really just crumbles back away when you make an
effort to dig it out. With a little extra elbow grease you could
certainly remove it cleanly when the need arose.

Maybe you could do that and then cover it with one of those 1 1/2 or
2-inch flooring transition strip -- either the uneven (thicker on one
side than the other like for carpet) or a completely level one that
works for transitioning between two different but level flooring
surfaced. (Sorry for my horrible terminology on the "flooring
transition strip;" it probably has a better name than that, but I'm nor
sure what it is!)