Do you know what they used to bond the tile in place? A cement based thinset as opposed to a mastic (note, thinset is a method, but people tend to think all thinset is cement based!?) does not re-emulsify and come loose. Also, most mastics are not specified for use on a shower floor (but some can be used on the walls).
The spec sheet on Fusion Pro says to not use it on joints greater than 1/2", which, is kind of hard to judge with that type of tile.
https://www.custombuildingproducts.com/TDS/TDS-242.pdf
The stuff cures by drying, so that's one reason why it can't be applied too deep or in too wide of an area or, if the underlying material is damp...if so, it will take much longer to cure. Under normal circumstances, you must wait 7-days before using the shower and probably longer with those larger grout joints. If you started showering prior to that time delay, that's probably your issue.
FWIW, a cement based material cures by a chemical process, and as long as it has the proper conditions, once cured, it's quite stable. Both Fusion Pro and a cement based grout are rated in the industry for 'full' strength after 28-days, and will probably slowly get stronger for awhile after that delay.
