S
Stef
You are correct, the adhesive is only designed as a gap filler but if I'm fixing to a wooden substrate then I use a flex adhesive as it sits better in my head.I agree that it is more likely the tiles. I also only use plasterboard tape and never had problems. The only thing I do different is use non flexible adhesive between the hardiebacker and the floorboards as I'm sure I read on the hardie website that ur supposed to use non flexi because it's to create a solid non flexible bed under the board and the main part designed to hold it down is the screws? Correct me if I'm wrong though?
Try ripping Hardie back out after it's been fixed & see if it's only the screws that hold it in place, it ain't budging.
Hardie is not a new product, been around 20years roughly, it may be a new product to some but not others.
I've never had a failure with Hardie & ive lost count of the number of boards I've fitted & tiled to.