I think it may be a temperature-related thing. I've had a Silva type 4 for many years that grows a bubble, usually in cold conditions. Doesn't affect the function (well, I ain't been lost yet).
I'm a balloonist and my Silva compass has been up to 10,000ft several times; we once went double that. Not only is air pressure low, but it is also very cold. It's got a whopping great bubble which is really annoying, but it seems to work OK - at present . . . . . . .
The non liquid compass mentioned above sounds the best way of not bubbling.
I've always put them down to age and/or temperature, never having used my compass at high altitude.
I use it like a circular spirit level (I have one on my camera tripod) to keep the compass level.
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