Idiomatic phrases usually (but not always) retain the stress pattern of their original syntax even after they lexicalize. See: “the CAT’s out of the BAG” vs. “the CLAM’S out of the POND” and “kick the BUCKet” vs. “beat the MONkey”.

So, while I agree with you that “time being” (and probably all of “for the time being”, for that matter) is idiomatic, its prosody has fossilized from its original syntax in which “being” modified “time”.