גופא אמר רב יהודה אמר שמואל:
,שלשת אלפים הלכות נשתכחו בימי אבלו של משה
"אמרו לו ליהושע שאל א"ל "לא בשמים היא
Rav Yehuda says that Shmuel says: Three thousand halakhot were forgotten during the days of mourning for Moses. The people said to Joshua: Ask (for guidance from Heaven so that you can reacquire the forgotten halakhot). Joshua said to them: “It is not in heaven” (Deuteronomy 30:12) (Once the Torah was given on Sinai, the Sages of each generation must determine the halakha. No new halakhot may be added or subtracted by heavenly instruction or through prophecy) Temura 16a.
A similar situation occurred to Moses, where he "forgot" Halochos or didn't know what to reply (Hamekoshesh, Bnot Zlofchad). However, he immediately asked G-d for assistance and G-d revealed the Heavenly Halochos.
NOTE: This question is only relevant to the approach that Moses received the whole Torah on Mt. Sinai during the 80 days, and not subsequently through the 40 years piece by piece. I'd be glad if you don't mention that in your answers.
While Moses was on a higher level of prophecy than Yehoshua, it seems that the level does not matter at all, they later relied/declined even on Bas Kol.
If the rule is so fundamental, why did Moses choose to ask G-d instead of inferring the Halochos from his existing knowledge?