Here's an underrated possibility - more and more voters think Biden will win, so they're more comfortable voting for downballot Republicans since they're less likely to enable Trump.
[citation needed]
Either way, I don't doubt that there are some Biden/Republican downballot voters, though I don't think they'll make up a significant portion of the electorate, and in most competitive races won't significantly outnumber Trump/Democrat downballot voters (and in some races, there could be more of these voters.)