The ninth b'racha of the amida begins with the words
בָּרֵךְ עָלֵינוּ ה' אֱ-לֹהֵינוּ
אֶת הַשָּׁנָה הַזֹּאת
וְאֶת כָּל מִינֵי תְּבוּאָתָהּ לְטוֹבָה
where both את are vocalized with segol (three triangular dots).
I have heard multiple sh'lichei tzibur at a Lubavitch establishment pronounce the second "אֶת" as "eis" (or [eɪs]). So I consulted two of the most common Lubavitch-used sidurim, and indeed it was printed that way, contrary to every other paper sidur of various versions and traditions I've checked, which have them both as "es".
[Assuming the word is the accusative marker "את" and not the homonymous word for "with"] the vocalization might vary based on the following word if it is following the Biblical pattern of vocalization. If so, which of the rules of that system would make these two words come out differently?
Here are a pair provided by @paquda where the "את" is either connected to the next word by a "-" and "shortened" from its default state of "eis" to "es" or disconnected and left in its natural state of vocalization:
כִּ֚י אֶת־כׇּל־מַֽעֲשֶׂ֔ה הָאֱלֹהִ֛ים יָבִ֥א בְמִשְׁפָּ֖ט עַ֣ל כׇּל־נֶעְלָ֑ם אִם־ט֖וֹב וְאִם־רָֽע׃
וַיִּבְרָ֣א אֱלֹהִ֔ים אֶת־הַתַּנִּינִ֖ם הַגְּדֹלִ֑ים וְאֵ֣ת כׇּל־נֶ֣פֶשׁ הַֽחַיָּ֣ה ׀ הָֽרֹמֶ֡שֶׂת אֲשֶׁר֩ שָׁרְצ֨וּ הַמַּ֜יִם לְמִֽינֵהֶ֗ם וְאֵ֨ת כׇּל־ע֤וֹף כָּנָף֙ לְמִינֵ֔הוּ וַיַּ֥רְא אֱלֹהִ֖ים כִּי־טֽוֹב׃
Why does the second "את" have a tzeirei ([ei])?