I am using sandwichable-1.1.2__1.16.2 and am adding my own cutting board recipes. If I set the input as a tag rather than item, the resulting recipes work - it takes any item with that tag - but in REI it only shows the first item in the tag as being applicable and so it isn't clear to players what items will or won't work.

That may be a problem with REI. I couldn't think of why that would be an issue on my end, but I'll look into it.

I have found the issue, it should be fixed by the next v1.2 beta.