The lumpy idle is VERY common with Skyactiv, especially with when the A/C compressor cycles on. My '17 CX does it, a loaner 2020 Mazda3 did it, my friends 2021 CX-5 does it, my bosses '17 Miata 6-speed manual does it. My *theory* is that at low engine speeds there isn't sufficient air swirl in the cylinder, and with unusually high compression + direct injection the air/fuel charge isn't very homogeneous at low speeds, resulting in somewhat uneven cylinder contribution and a somewhat sooty exhaust (most skyactiv vehicles I've looked at have soot build up on exhaust tips). The lumpiness seems exacerbated by load (A/C compressor and trans in D or R).