Yetziv Pisgam and Chabakuk

Question

One common Ashkenazi minhag on Shavuos is to recite the piyut (poem) of Yetziv Pisgam on the second day of Shavuot after the first Passuk of the Haftorah from Chabakuk. The first letter of each line forms an acrostic, spelling out the name of the author, Yaakov BeRabbi Meir Levi, who is commonly identified as Rabbeinu Tam.

I understand that Yetziv Pisgam is for the meturgaman (translator) to ask permission to read his translation of the Haftara and Shavuos is a time to ask permission just like Moshe asked permission to give the Torah on Mount Sinai see here.

My question: What specific link is there between the Piyut and the Haftara passage from Chabakuk?

After all, this Piyut could have been said on the first day Shavuos during the Haftara of Yechezkel just like the Akdomos are said then.

Answer 1

Some quick Otzar Hachochma searching has revealed the following:

Tessler's Minhagei Chag Hashavuos (Anaf B, 1) notes that Yetziv Pisgam has nothing in particular to do with Shavuos, and in fact, in some communities it was recited on the 7th day of Pesach. If this is the case, there is no reason for why we read it on the second day specifically...

On the other hand he ignores the Levush (OC 494:2), who connects Yetziv Pisgam which mentions names of Malachim to the Ma'aseh Merkava, and since the The first Passuk of the Parsha of Chabakuk is referring to Ma'seh Merkava which is why it is read on second day Shavuot, so we read Yetziv Pisgam as well after the first Passuk:

ומפטיר בחבקוק מן ויי בהיכל קדשו עד למנצח בנגינותי שהוא ג״ב מעשה מרכבה
סימן ת״צ סעיף ט ומזכדין נשמות כרפ״ל
ונוהגין לומר יציב פתגם (בפוזנא אין אומרים) אחר פסוק ראשון שיש בו ג"כ מענין המרכבה והמלאכים.‏

According to Levush, it would seem to fit in nicely on the first day as well.

However, in Takanas Hashavim to OC 494 Taz 1, he notes what should be a good answer according to the Levush and others, namely, that Tosefta Megillah 3:19 implies that even according to those who permit reading the Ma'aseh Merkava, we would not translate it publicly. If so, there was no translation of the Haftarah on the first day of Shavuos, and therefore, no permission was asked before translation. Whereas the second day since the Merkava has already been mentioned in the first Passuk, the Targum can be said on the rest of the Parsha from then on.