If you look at the various options for tile over wood floors in the TCNA Handbook, every option calls for some kind of underlayment on top of the subfloor. Engineered backer board products on a floor aren't going to debond from ceramic tile like yours unless the installer was grossly negligent. Pulling up the tile will decide that possibility immediately. If the thinset beneath has squished blobs of thinset or uncollapsed ridges of thinset, then you have a workmanship problem. (In the case of stone tile, a failure to back butter is a poor workmanship possibility, but your images clearly establish that yours is ceramic tile.)
Instead of an engineered backer board beneath your tile, there could be a plywood underlayment. The TCNA Handbook does have plywood underlayment options, but successfully implementing one is a little trickier than the engineered backer board products:
- The underlayment plywood must be exterior grade so that the glue isn't susceptible to water and
- there's a special thinset for applying tile to wood underlayment. This thinset must satisfy ANSI 118.11. It'll say so in the fine print if you can't find the right stuff from "good for wood!" marketing material.
Using the wrong thinset for plywood underlayment is an easy mistake to make, where this would probably be your best-case-scenario. Spot fixing tiles as they fail with an ANSI 118.11 thinset will solve this problem eventually.
There's nothing in the TCNA Handbook for tile directly over subfloor. In all likelihood a subfloor's plywood isn't going to be exterior rated, making it susceptible to water. A subfloor typically will be designed to the International Residential Code's maximum deflection of L/360, where that L/360 is obviously inadequate or else there would be a TCNA Handbook recipe for tiling a naked subfloor (put on some underlayment, subfloor--have you no decency?). I suppose it's possible that a specific naked subfloor is much stiffer than the International Residential Code's L/360 (3/4" OSB subfloor over a 16" joist layout, for instance). Maybe the exterior grade glue isn't actually that important. It's possible that spot fixing debonded tiles with an ANSI 118.11 adhesive could fix a problem with tile debonding from a plywood subfloor.