as Romain said: when LotW team make RDA available in TQSL and display it in LotW, it will be mandatory.
So treat it as exception based on tradition. I have not looked at it, but I guess LotW accpetance predates both mydx.eu and hamlog.
On the other hand, mandatory rule on RDA entry in HAMLOG helps people pay more atention on entering it.
Back in 2018 I've run in International Grid Chase. 12 one month long contests, and only LotW confirmed QSOs with UL vere valid.
Some 5% do not have UL entry. Similar story. Quite an effort to tell people how to fill up location in TQSL.
Years later there is much less LotW entries without UL.
Best 73, DSW