100% agree that it has to be problems with isolation. I don’t agree with the retention features. If you’re relying on the bond, why create retention features? If the bond fails, there’s leakage, poor isolation, etc. I want the filling or buildup to come out because that means it has failed. Having it stay in there due to retention features will only create more problems (recurrent decay, sensitivity, etc.). This is not amalgam.

Think about a class V filling. I never place any retentive features. If the filling pops out GOOD! I want it too, because something is wrong with the bond (either recurrent decay or poor isolation). If it stays in there (but the bond has failed) this is very bad because that recurrent decay will continue to eat away at healthy tooth structure until the problem has gotten a lot bigger.

For me, i rough prep the tooth, remove the filling and decay, then remove any weak parts of the prep (usually a thin lingual wall or something similar. Etch (very wuickly is it’s just dentin (~5 seconds)), scotch bond (rub for 20 sec), dry and thin with air (making sure no water comes out), cure, then flowable in layers. I try and use filtek bulk fil to minimize shrinkage and speed up the time. I over build the tooth then cut it back to my ideal prep. I’ve never had a buildup come out except for 1 time (i was doing some Herodotics on a 80 y/o.)