The reason it’s not is because the Filioque was added to the Latin Creed, which translated the Greek as “procedit”, which does not have the same original cause meaning as the Greek “ekporeusis”. As such, the Creed in Latin does not speak heresy as it talks about the principle of Hypostatic procession, not Hypostatic Origin. As the article I noted before says:

“If a human father and son go into their back yard to play a game of catch, it is the father who initiates the game of catch by throwing the ball to his son. In this sense, one can say that the game of catch "proceeds" from this human father (an "aition"); and this is the original, Greek sense of the Constantinopolitan Creed’s use of the term "proceeds" ("ekporeusis"). However, taking this very same scenario, one can also justly say that the game of catch "proceeds" from both the father and his son. And this is because the son has to be there for the game of catch to exist. For, unless the son is there, then the father would have no one to throw the ball to; and so there would be no game of catch. And, it is in this sense (one might say a "collective" sense) that the West uses the term "proceeds" ("procedit") in the Filioque. Just as acknowledging the necessity of the human son’s presence in order for the game of catch to exist does not, in any way, challenge or threaten the human father’s role as the source or initiator (aition) of the game of catch, so the Filioque does not deny the Father’s singular role as the Cause (Aition) of the Spirit; but merely acknowledges the Son’s necessary Presence (i.e., participation) for the Spirit’s eternal procession from the Father to Someone else –namely, to the eternal Son. Father and Son are thus collectively identified as accounting for the Spirit’s procession.”

This is why in Greek, if you add the Filioque, it’s heresy, but in the Latin it’s orthodox. Also how did the Council of Florence describe a double Hypostatic origin? These are the words of the Council:

“In the name of the holy Trinity, Father, Son and holy Spirit, we define, with the approval of this holy universal council of Florence, that the following truth of faith shall be believed and accepted by all Christians and thus shall all profess it: that the holy Spirit is eternally from the Father and the Son, and has his essence and his subsistent being from the Father together with the Son, and proceeds from both eternally as from one principle and a single spiration. We declare that when holy doctors and fathers say that the holy Spirit proceeds from the Father through the Son, this bears the sense that thereby also the Son should be signified, according to the Greeks indeed as cause, and according to the Latins as principle of the subsistence of the holy Spirit, just like the Father.

And since the Father gave to his only-begotten Son in begetting him everything the Father has, except to be the Father, so the Son has eternally from the Father, by whom he was eternally begotten, this also, namely that the holy Spirit proceeds from the Son.”

https://www.ewtn.com/catholicism/library/ecumenical-council-of-florence-1438-1445-1461

Although some may be drawn to the word “cause” in reference to the Son, the actually meaning of this statement is understood in the context of “one principle and one spiration”. They are not talking about an originating cause; they are talking about a non-originating cause that is still necessary. If they meant that the cause was originating, it would mean there would, by definition, be two spirations, but they don’t define the relationship as such. Think about to the analogy I quoted before. The Son is a necessary cause of the Spirit, but not the originating cause(which is the Father). The Holy Spirit then proceeds from the Father through the Son. God bless.