Hello @BeLo , i just came across your thread and i think that the issues are because of the “open” acrylic SlimTip mold , where the receivers ar not enclosed in the acrylic itself.
Due to this there is a tiny leak of high frequencies from the receiver housing itself into air and outside the mould. This causing feedback and the feedback manager going strong. Perhaps the acrylic mold itself reflecting the leaking frequences from the receiver housing to the outside, whereas the silicone mould does not so much reflecting.
With enclosed receivers into the mould the sound leakage from the receivers housing would be stopped. See symbolic pictures ( courtesly of @Volusiano )
Mold with receiver sticking outside , non sealed receiver
Some sound is leaking to outside , due to resonating receiver housing
Enclosed receiver into mold ( courtesly of @Volusiano )
lesser or no sound leakage into outside, due to fully sealed receiver.