Welcome to the world ND1 ownership (a 2015 is an ND1, ND2 started 2018/19). It’s highly likely to be right, known failure point on the car is the rear hub carrier bushes and Mazda don’t sell them as a standalone part so you have to replace the entire hub carrier if you do it OEM which cost over £400 per side without labour.
None OEM route is the 27AL blue Polybush set MX5 MK4 (ND) Polyurethane Suspension Bushes & Components - Polybush
Hearing the knocking from the rear going over rutted surfaces like this is a typical symptom REC_5340_cut