Rewrite fails because variable ?o is considered unbound during rewrite of filter expression

[Aklakan]

Select Distinct ?s {
    ?s ?p ?o .
    { Select Distinct ?s {
      ?s <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/ns/dcat#Dataset>
    } }
    Filter(Regex(str(?s), "o") || Regex(str(?o), "o"))
} 

For some reason, the filter Regex(str(?o), "o")) is applied on mapping that only defines ?s.
A likely reason is a bug in the filter placement optimizer.