In this scenario:
- There is a possible range access for table T
- And there is a ref access on the same index which uses fewer key parts
- The join optimizer picks the ref access (because it is cheaper)
- make_join_select applies this heuristic to switch to range:
  /* Range uses longer key;  Use this instead of ref on key */

Join buffer will be used without having called
JOIN_TAB::make_scan_filter(). This means, conditions that should be
checked when reading table T will be checked after T is joined with the
contents of the join buffer, instead.

Fixed this by adding a make_scan_filter() check.
(updated patch after backport to 10.3)
(Fix testcase on Windows)