Dak Man
The issue is the amount of protrusion of the heater matrix tubes above their support plate within the FOHE. These are notoriosly difficult to manufacture and the protrusion height is largely driven by their method of attachment, i.e. brazed joints.
If they protrude above a certain dimension then it is likely that any upstream release of a "snowball" will not pass cleanly through the FOHE and will "stick" or "cling" at the FOHE heater matrix inlet plate and hence restrict fuel flow.
The solution is to provide a bypass within the FOHE, similar to that provided for Fuel Filters, which is where, I believe, RR are heading.
The problem is that the current EASA fuel icing cert requirements do not include testing with fuel at a temperature conducive to "sticky ice", ("Snownballs" or "Sticky Ice" occur in fuel at or around -8°C). However sticky ice does not seem to be an issue if, as above, heater matrix tube protrusion is below a certain value.
Good explanation of what appears to us (out of the direct loop) a plausible explanation of the uniqueness of one installation vs the rest.
Now if only the continued new posters would take note of these kinds of explanations before asking the same questions over and over
