And another desperate attempt of getting some meaningful values: I replaced the above SimConnect_RequestFacilitiesList call with SimConnect_SubscribeToFacilities.
Oh… wait!
Now that is interesting: while I initially (right after the call) do get the same Same Airstrip (HTSE) 30 times after that (like after 10 seconds or so, have to verify again) I actually do get meaningful results!
I have yet to verify those returned ICAO codes (I just received LFHE, which, yes, makes way more sense, given my current location) and as I “slew” over the landscape I do get more entries (like one more at a time, not a “burst” of 30 entries).
→ Could it me that some “debug / test code” was left in production code? 
Anyway, at least that gives us something to work with (as a workaround I will now stick with the “subscribe” variant and simply ignore any entry with ICAO HTSE - which implies that my software won’t work at the Same Airstrip - but oh well ;))