Yeah, it's subtle because you're at low RPM but I think it's going into 5th either to climb a hill or for slight engine breaking (what you feel when it slows for a second).
Try this: jump on the highway or wherever you can cruise at 50 like usual. Turn cruise control on and swap to S then hit the + button to make it go into 6th and watch to see if it still does it... As far as I know if it's in S+ the cruise control will not up or downshift by itself unless it *really* needs to. If the problem persists I'd say take it to the dealer.
'12 Sterling Grey Titanium Hatch - Moonroof / Winter Package / DCT / 17' Alloy Wheels
- Mods: MazdaSpeed3 RMM, Steeda Rear Sway Bar, Steeda CAI, Steeda Springs, Koni Shocks, ST Struts, FSWerks Stealth Exhaust, Euro Tail Lights