In his disagreement with the Maharshal's ruling not to start Shemini Atzeres early, the Taz's first objection is a long poetic rendition about why we shouldn't forbid something that isn't mentioned in the Gemara or earlier Poskim.
On Shavuos, not only does the Taz accept the Shelah's ruling not to make kiddush early, but he extends it to not davening maariv early. As far as I know, the whole issue isn't mentioned by anyone earlier than the Shelah, and maariv isn't mentioned by anyone earlier than the Taz.
Why is the Taz inconsistent between these two very similar cases?
(I'm asking only about the Taz's objection to forbidding something that was not mentioned in earlier sources. His further objections to the Maharshal are based on his opinion that can be summarized as "strong tosefes". This is conceptually consistent with his strict ruling on Shavuos once you accept the Shelah's drasha of תמימות. I don't think this point is essential to the question, but can elaborate if necessary.)