Not quite sure (I suspect it's a bug), but it seems like it has to do with the implementation of make-readtable (which brackets.scm uses).
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> (parameterize ((current-readtable #f)) (read))
x`y ; read in as two items
> (parameterize ((current-readtable (make-readtable #f))) (read))
x`y ; read in as one symbol