Horizontal tiles held on with Plumber's Mait?

I've just started pulling off the silicone sealant around my bath cos it's come loose in places and mould etc.

Turns out the 4 thin strips of tile that sit horizontally behind the taps were actually held in place by the silicone! So they have now come off in my hand. The original plain white tiles are underneath (I noticed earlier this year that the brown tiles in my bathroom are on top of old white tiles!).

Anyway, it looks to me like these 4 brown cut-down tiles were just sat on top of the white tiles with plumbers mait? It was dried out and crusty (not sticky like new plumbers mait), but it was still soft and didn't have any adhesiveness to the tiles at all really.

I'm just guessing that this is plumber's mait? can someone please confirm, as I have a tub of it in my flat anyway and can just do the same job with it again once I've fully cleaned all surfaces.

This is the first time I've ever touched anything to do with tiles, and always assumed the adhesive between tiles would be rock hard and not removable unless tiles smashed off.

thanks (whenever I try and do any simple DIY task it always leads on to something else needing doing in the process!!)

Tiles are installed using tile adhesive, not plumbers mait or silicone!
When installing tiles over tiles, the base tile should be keyed and primed.

I appreciate that's not how it's meant to be done. I'm just trying to figure out what the hell this soft brown stuff was. Is using plumber's mait a common bodge job for this? My flat is full of bodge jobs seriously.

Never come across or heard of plumbers mait being used as a tile adhesive, until now!