The spec says:

Additionally, drivers MUST unpin a ClientSession when any individual commitTransaction command attempt fails with an UnknownTransactionCommitResult error label. In cases where the UnknownTransactionCommitResult causes an automatic retry attempt, drivers MUST unpin the ClientSession before performing server selection for the retry.

And MaxTimeMSExpired is defined as an UnknownTransactionCommitResult error. So a MaxTimeMSExpired error when running commitTransaction should unpin the session. However the "remain pinned after non-transient error on commit" test asserts the opposite behavior; that a session remains pinned after a MaxTimeMSExpired error on commit.