Here's the code in question: https://github.com/mongodb/mongo/blob/r3.6.14/src/mongo/db/repl/sync_tail.cpp#L335-L351
That should be doing something similar to https://github.com/mongodb/mongo/blob/r3.6.14/src/mongo/db/ops/write_ops_exec.cpp#L122-L124
This behaviour is called out in https://docs.mongodb.com/manual/release-notes/4.2/#slow-oplog, though I don't see any discussion on
SERVER-32146 or DOCS-12178 for whether sampling was considered.
There's also a couple other places where this might need to be checked:
It's possible a better fix would be to decouple slowOpSampleRate and slowms, such that you log all slow operations but then subject non-slow ops to sampling.