Try removing hack added in nhibernate#2990 (see 32fdfec)

Author: bahusoid

src/NHibernate/Hql/Ast/ANTLR/HqlSqlWalker.cs

@@ -839,8 +839,6 @@ void CreateFromJoinElement(
 					// or on an association join in subquery from a parent entity in parent from clause
 					// like in "from EntityA a where exists (from EntityB join a.Assocation)"
 					fromElement.FromClause.AppendFromElement(fromElement);
-					if (fromElement.IsImplied)
-						fromElement.JoinSequence.SetUseThetaStyle(true);
 				}
 			}