So, my question is: has anyone seen this phenomenon before and know of a solution to stop further damage from occurring?
It common Mark. We simply call it 'evaporation' when the bottle is unopened. Also, as already mentioned do not store whisky horizontally. Even if the liquid has leaked out because of capillary attraction, its always referred to as evaporation.
Occasionally the cork is not as tight as it could be, and it's simply a lottery as to which bottles will suffer from this. Don't worry about the oxidation (oxidation concerning whisky in the bottle is no more than just talk and conjecture anyway). Evaporation will affect the bottle value, but not always, and yours is just at the shoulder -- nothing to worry about. In fact, that hole in the capsule may affect the value more as it may put some collectors off.
So, to drink it, sell it, or just keep it. Drinking it is the easy one, just drink it now or soon
. If you want to sell -- the buyer will need to know a detailed description, and will appreciate photos, particularly both the capsule damage and the fill level. If you want to keep it you could try Parafilm M (it's an American product but you should find it on eBay or in a model shop). Similar to clingfilm but thicker and grips when stretched, as opposed to 'cling'. Make sure you cover the entire capsule, and you could apply it to any other long term stored bottle just as a precaution. I don't know if it's proven to work but it's the only real option i know of, or you could dip it in wax which has it's own pros and cons.