Right now dispenser block placement behaviors do not get registered if another mod registers a behavior for the same block.
I'm not sure how (and if) this could be solved since block placement usually should have lower priority than other behaviors (hence why it was probably chosen not to register if another one was present) so I'm guessing it should first try to call previous registered behaviors to only then, if they lead back to the vanilla shoot behavior ( meaning they failed ), perform the block placement action

Hello! Quark's moved to 1.18 now-- unfortunately, this means that (unless your bug is critical and/or game-breaking), we're closing support for issues filed for versions prior to it (e.g. Quark on 1.16).

If your issue still occurs on 1.18, or if your issue happens to be rather critical (e.g. consistently crashing the game with no other mods present), please leave another note or open a new issue. Thanks for your cooperation!

How is this relevant?

Well for example I tried to register new optional behaviors for some blocks. Those behaviors had some conditions to them and would, for example, check the block in front and in case they would fail they would just fallback on the previous registered one (In this case the vanilla shoot behavior). This meant that it was impossible to place those blocks using Quark block placement feature since if my behaviors were to get registered first, quark would not register its ones. I tried thinking of workarounds for this but I didn't manage to find a clean solution so I figured it might be better to ask here.