Just checked, and I DID NOT get the PS4 version of Bugsnax along with the PS5 version. My bad. I've been conditioned to think cross-buy titles were automatically on all platforms with Plus, because, well, they almost always were. I get why this would be PS5-only though.
The PS Plus version was announced as PS5 only.
But yes, cross platform titles are petty much both put on PS plus so it's understandable you would have thought so.