This is an expected e3k behavior.
MU-MIMO grouping depends on angle diversity between SMs. If SMs are clustered in some area they are not groupable. Refer to the following topics for detailed explanation.
ePMP 3000 4x4 MU-MIMO performance - Knowledge Base / ePMP - Cambium Community (cambiumnetworks.com)
ePMP 3000 MU-MIMO grouping Criterion - Knowledge Base / ePMP - Cambium Community (cambiumnetworks.com)