Some of our code sets binVersion for each node in ReplSetTest options.

Here we take our node-specific options and merge them into options object:
https://github.com/mongodb/mongo/blob/05518f9b480f4f65fc963b47ef4cf45c9e4807b5/src/mongo/shell/replsettest.js#L2782

And in the next code block we override binVersion with "latest" or useRandomBinVersionsWithinReplicaSet: https://github.com/mongodb/mongo/blob/05518f9b480f4f65fc963b47ef4cf45c9e4807b5/src/mongo/shell/replsettest.js#L2795

We also state that setting both binVersion and useRandomBinVersionsWithinReplicaSet is incorrect, but we check that before we merge node-specific options:
https://github.com/mongodb/mongo/blob/05518f9b480f4f65fc963b47ef4cf45c9e4807b5/src/mongo/shell/replsettest.js#L2754

We also set FCV based on node-specific binVersion values: 
https://github.com/mongodb/mongo/blob/05518f9b480f4f65fc963b47ef4cf45c9e4807b5/src/mongo/shell/replsettest.js#L1363
https://github.com/mongodb/mongo/blob/05518f9b480f4f65fc963b47ef4cf45c9e4807b5/src/mongo/shell/replsettest.js#L1435

Which leads to an interesting situation when, for example, update fuzzers think they compare 6.0 with 6.2, but actually they compare 6.2 with and without 6.0 FCV.

It would be great if we can change the behaviour of useRandomBinVersionsWithinReplicaSet to use binVersion instead of “latest” and allow setting both fields.

We have gone with the solution suggested in the comments. The above will be done in a separate ticket: SERVER-71320