Unfortunately, we can't see stats like Z-Contact% and O-Swing% (the bright red flags for Riley even when he was getting great results) splits against LHP/RHP.
We can get some useful bits of split data though:
K% vs RHP - 36.0%
K% vs LHP - 37.8%
BB% vs RHP - 3.6%
BB% vs LHP - 10.8%
BABIP vs RHP - .286
BABIP vs LHP - .323
HR/FB vs RHP - 18.0%
HR/FB vs LHP - 33.3%
Needless to say, his HR/FB rate and probably some of his BABIP vs LHP is completely unsustainable. Remove 3 of his HRs vs LHP to simulate a more reasonable 19% HR/FB rate, and his slash line vs LHP drops to .215/.297/.462...an OPS of .759.
And that isn't even regressing his .323 BABIP at all...he simply doesn't make enough contact.
I don't think Riley is the answer to any question at the MLB level at the beginning of 2020, not even vs LHP. He has a pretty good track record of making adjustments, so it would be foolish to give up on him, but he shouldn't be relied on to fill any role on a contending club right now.