Normaly the logic would be that it first tries to use the category icon (e.g. temperature, firstfloor etc.). If no category is defined it tries to take the icon configured in your item config. If nothing is found/configured there, it takes the first char of the item-name as the icon representation (so it should be an ‘H’ in your case - given that nothing else is available).
The only thing I could imagine is, that you’re either using an unknown category with no icon-representation available or have an icon defined in your config which can’t be found.