The query:

let $Q{}foo:=1
return $Q{}foo

produces a syntax error, unless it is amended to:

let $Q{}foo :=1
return $Q{}foo

(i.e. space inserted before the colon.) 

This first occurs with BaseX 12.1 and with earlier versions did not produce a syntax error. For comparison, this syntax error is not observed under Saxon XQuery 12.9 or 13.0, or (with Saxon or BaseX) if the variable is a QName instead.

Many thanks,

Andrew