I'm sure there are many second party studios Microsoft use that they don't own.
Here's a list:
Certain Affinity - (Halo 4, Halo: The Master Chief Collection & not released anything in two years)
Iron Galaxy - (Killer Instinct but recently made a 2018 multiplat)
Armature - (Recore)
Playful - Super Lucky's Tale
Creative Assembly - Halo Wars 2(Sold poorly so unlikely)
Dlala Studios - Battletoads (Not done yet)
Remedy Entertainment - (Made Quantam Break but busy with a multiplat now)
Ruffian Games - Crackdown 3 (Not done yet)
Sumo Digital - Carckdown 3 (Not done yet)
Moon Studios - Ori & The Will Of The Wisps (Not done yet)
Asobo - Recore
Skybox - Halo Infinite (Not done yet)
Relic - Age of Empires IV (Not done yet)
They also use Frontier a decent amount,did Screamride,Zoo Tycoon and a bunch of kinect games.
Ah yea but they made Jurrassic Evolution recently which was big hit so I doubt they'll avoid doing a sequel to work with Microsoft but who knows.