Hello,
I am surprised that nobody noticed this minor but annoying bug in HALion so far - at least, I have not found any bug report for it…
When you set a High Key on the Key Range Page (in multi-instrument mode), then the highest note is visually excluded from the selected key range on the main keyboard - as you can see in the screenshot, on the bottom keyboard, note C5 is greyed out:
Fortunately, the playback works as expected (C5 still plays), just the visual indication of the key range is shortened by excluding the highest note.
This minor bug could be resolved easily, I believe 
Best regards
Miloslav