On the first day of using FMPA184.108.40.206/Mac with a file hosted on FMS 220.127.116.118/Windows with ODBC connections to an MS SQL database, I experienced erratic behavior with a relationship (which I have never experienced with FMPA 14 or 15).
An initial investigation of the script step Set Variable ($countRel; Value: Count(EA7_RELATIONSHIPS::PARENTID) whose correct value, in a particular instance, should have been 3 (i.e. 3 related records), was 0. However, running the script again in the same exact situation, did render 3. In other words, it gave different values at different times. The step is part of a script that obtains (i.e. updates) record IDs from the MS SQL data and the erratic behavior left many of them blank/undefined.