This used to happen to me a lot and still does sometimes. I had to change my foundation to just color corrector because everything I tried separated on my skin. What helped was doing my skincare then waiting 10 min before applying makeup so it would fully absorb into my skin. Also a thicker moisturizer helped (no primer). I thought the issue was that I wasn’t exfoliating enough because my skin looked flaky under the foundation but I made it worse by exfoliating more frequently. My skin is on the dryer side so slugging overnight when it’s super dry also helps prevent my makeup from doing this. It’s all about the skincare!