Basically feedback is when the sound from the HA receiver (speaker) makes its way back to the HA microphone. In a BTE the sound would have to come from the speaker, go down the tube, into the ear canal and then go back to the speaker. It generally can go one of two ways. One is simply through the vent, which most molds will have. The other can be leaks around the mold if the mold does not fit well, of if the fit changes when you move your jaw or talk. Is there any pattern to when you hear the feedback? That might be a clue.
I recently had molds made and they have been less than satisfactory. They were done with a relaxed jaw while the material setup. The next set which I am waiting for was done with an open jaw, which I hope will fit better. They are silicone which is claimed to be better for feedback. Mine are a simple canal type.
I did a bit of research on these things and one of the claims I found was that the true sealing was done in the ear canal, not in the outer ear. If the mold does not seal in the ear, the the outer half shell will not stop it from getting back to the microphones.
I’m not sure what to make of your description of an additional tube. I had saved a link to this Starkey Guide to Earmolds and they talk about a power vent, which sounds like a tube on the end of the vent which is supposedly routed to suppress feedback. See page 10 of this guide.
The other issue besides fit and lack of sealing is that the vent is too large. Generally the larger your loss at 3-4 kHz the smaller the vent has to be to prevent feedback.
But to answer your question, if you have feedback then go back for resolution. I got mine at Costco and they claim they will remake them as many times as necessary to get them right.