Shame any comments on the actual build are this low down. Again, not doing this to shit on OP but just for everyone's general knowledge before they do something similar.

While OP says it was a "test", I can almost guarantee this is going to fail (how soon? who knows) due to a few glaring errors (and to your question, you should indeed have concrete as your base and then insulate, not vice-versa. Ditto on the concrete board).

Foam on sand - not rigid or dimensionally stable is likely to be the first cause of death as things settle. If not that, then likely due to the whole thing lacking any waterproofing. Water will permeate the grout/tile (expected in all showers), but without any waterproofing, it will then soak into the concrete, which will saturate, and then saturate into concrete board (And thus will be trapped by the plastic) and then ultimately fail.

Again, not really so much of a question of "if" but a question of when. With minimal use it could be a long ways off. But I'd bet the foam on sand base will be the issue first.

And bonus sketchy points for in floor heating in the shower without waterproofing and the above mentioned water sponge created with the plastic. Should actually be a risk as long as floor is dimensionally stable and cable doesnt get stressed due to failures/shifting...(see above)