Let me quote the person who send me the bug originally:

"The MySQL backend returns "int" values for boolean properties, while the
AnyDBM backend returns "bool" values.  For example:

  db.myclass.get(id, bool_prop)

will have different types depending on the back ends, at least in our
version of Roundup.  That seems undesirable; returning bool values
everywhere would make more sense."

HTH,
     Stefan