> For example, Java thread execution order is UB.

In this context "UB" means something different than how you're using it. The UB being mentioned here is the "nasal demons" form, i.e., programs which contain undefined behavior have no defined meaning according to the language semantics.

What you're talking about is probably better described in this context as "unspecified behavior", which is behavior that the language standard does not mandate but does not render programs meaningless. For example, IIRC in C++ the order in which g(), h(), and i() are evaluated in f(g(), h(), and i()) is unspecified - an implementation can pick any order, and the order doesn't have to be consistent, but no matter the order the program is valid (approximately speaking).