I've made some tests using Malta 42 scenario. In fact, I've altered the scenario and placed all coastal guns inside the fortified places near Valletta (attached), to make it easier to test.
If I move an italian vessel around and it gets interdicted by some coastal units, these are ALWAYS eliminated by counter fire. The vessel can eliminate all coastal units within the fortification after being interdicted 4 or 5 times, only using counter fire.
In contrast, if I keep the vessel in place and use direct bombardment against the same group of coastal units, there is almost no effect. Most coastal units can survive the ten rounds available in the turn. This is, in fact, consistent with what happen with the other ground units, so it must be ok. I'm not evaluating historical plausibility, just consistency.
The bug must be present in the way combat resolution is done when counter firing, after interdiction.
The effect is clear. Just do some tests allowing a single vessel to be interdicted multiple times during a turn, then reload and just try to spot and bombard the coastal units with a static ship. The results are almost perfect opposites.