The same Taz that says that you don’t eat in the succa once you made an early Shmini Atzeres, that same Taz says you can’t daven Mariv early on Shavuos night. The Taz holds that making Shabbos or Yom Tov early only affects the halachos of Shabbos and Yom Tov, but it’s still still Friday or Erev Yom Tov in regard to Mila, hefsek tahara for a nida,etc. Therefore if you’d make an early Shavous you wouldn’t be allowed to do melacha, but it’s still daylight outside so you wouldn’t have Temimos. That’s why he says you must wait until Tzeis to daven Mariv.
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