The enumerated type has three values in its domain: BITMASK_1, BITMASK_2
and BITMASK_3.   A value such as BITMASK_1 | BITMASK_2 is not in the
domain of the enumerated type.

Yet the code uses that type to represent it:

   Bitmask x = BITMASK_1 | BITMASK_2;

What that means is that x now has a value which is not one of the declared
constants, and therefore outside of the type.

Note that this was banished from C++ long ago, because C++ was given
type-safe enumerations.

That's one good reason to write your C in such a way that it also compiles with
a C++ compiler.

You cannot use enumerations for bitmasks, period. It's conceptually wrong,
and banished by the better dialect of C known as C++.