Its been ages I've heard about glycerin (or any fluid) filled dial instruments !
The purpose is to dampen the needle movements.
Of course if an air bubble escapes into the dial area - it will sort of totally damage the view on the dial
The instrument will have the filler hole at top position - when they are fitted. This is to avoid longterm leaks.
Meaning if you tilt the instrument more than the car will do during normal drive conditions - there is a risc of leaking.
And thats about it. It must of course be possible to remove the cluster allowing for any service - like bulb changing - or electric gremlins.