“So too, bhikkhus, as to those recluses and brahmins who are unskilled in this world and the other world, unskilled in Māra’s realm and what is outside Māra’s realm, unskilled in the realm of Death and what is outside the realm of Death—it will lead to the harm and suffering for a long time of those who think they should listen to them and place faith in them." - Cūḷagopālaka SuttaBy the way, Theravada Buddhism does not vouch for the authenticity of the teachings of other schools. So if you leave one Zen teacher and start following another Zen teacher, it wouldn't make much of a difference.
I too have a similar issue myself where that what I "knew" and have been taught as Buddhism and Buddhas teaching in reality was actually NOT actually the Buddhas teachings but later Buddhist teachings. Because of this, I have been mislead by a specific sect of Buddhism teaching that which is not the Buddhas words as if they are the Buddhas words. This has/had led me to become confused as to what is the truth as spoken by the Buddha because Buddhas teachings has been distorted and mixed with what is not Buddhas teaching.Do not get confused as to if I am speaking ill of this sect, it is not that, it is just that I am more inquisitive as a person and was always able to attempt to fact check what was taught and to inquire whether it relates to what Buddhas taught.Essentially what i was taught is a different presentation to the original view of Buddha, or more specifically the traditional theravada presentation.I would suggest you look into these sources for guidance while keeping in mind that we as individuals need to also work with the tools we have as an individual meditator.Skillfull means (upaya) is also a tool that we can use and if other presentations do not actually deviate from what the Buddha taught they may be employing skillfull means in their presentation.Kalama sutta:https://www.accesstoinsight.org/lib/authors/soma/wheel008.htmlAN 1.132–139“Those monks who explain what is not taught by the Buddha on monastic training as taught by the Buddha … what is taught by the Buddha on monastic training as not taught by the Buddha … what was not spoken and stated by the Buddha as spoken and stated by the Buddha … what was spoken and stated by the Buddha as not spoken and stated by the Buddha … what was not practiced by the Buddha as practiced by the Buddha … what was practiced by the Buddha as not practiced by the Buddha … what was not prescribed by the Buddha as prescribed by the Buddha … what was prescribed by the Buddha as not prescribed by the Buddha are acting for the misery and unhappiness of the people, for the harm, misery, and suffering of gods and humans. They make a lot of bad karma and make the true teaching disappear.”Specifically the stanzas of AN 1.130 and AN 1.140