While MU-MIMO does work for flexible mode, when it was released in 5.10, its status was that of “Performance optimization is pending for Flexible TDD ratio configurations.” I’m not sure how much has been optimized since then in the newest release, 5.10.3. Eventually it will be fully optimized as it was with e3k. There are a bunch of MU-MIMO performance optimizations planned for the upcoming 5.11 firmware release. You can read about this and more in the official firmware new feature release post:
That being said, I don’t think you’ll be able to see MU-MIMO gains using the RFE horn antenna arrays. When we tried using the RFE 4x4 horn array using a ePMP 3000, we found that it did not work well at all. You can read more about my experience HERE.
You could temporarily try TDD fixed mode to see if it makes a difference in triggering MU-MIMO.