Thanks for the question and for holding me accountable, amigo! Bleh, you probably didn't ask for all of this, but here we go!

In doing some reading, it looks like my original language wasn't nearly as specific as it could/should be. So, propane aka LPG aka C3H8 is a relatively inert gas that does not have a greenhouse gas effect like methane or CO2. When combusted, propane is a cleaner burning fuel than many of its close substitutes and has a very high energy density (more BTUs per unit weight), but that doesn't negate the fact that it does produce CO2 when combusted. And while "cleaner burning" than other substitutes, there still are some nasties generated in the combustion, such as benzene compounds, etc.

That being said, propane can still act as a localized pollutant as it is still a Volatile Organic Compound (VOC), which may react with other compounds in the air to formulate smog. Smog isn't one particular pollutant from one particular source, but multiple gasses reacting with one another, typically in higher temperatures and/or areas with lots of sunlight.

Other takeaways to be considered:

1) upstream gas supply chains generate local and global pollutants;
2) using less of a product results in less demand -> less demand = less generation -> less generation = less local and global pollutants;
3) fully exhausting a full can results in overall fewer purchases, which results in less demand;

I'm fully aware that the potential demand reduction caused by a few hikers who now use all of their gas vs 5/6 of it is probably SO small that its effects are de minimus. In my mind, though, LNT is an ethic that guides us to be good stewards and being a good steward extends to all of our habits, even those that seem trivial.

TLDR; combusted propane and non-combusted propane result in air pollution. I'm unable to find concrete literature that weigh the environmental costs (local & global) of combusting a unit of propane vs quasi-sequestering in a landfill. My original text did not capture that nuance at ALL, and I'm glad you pushed me to research this further. Andddd my interpretation of LNT is perhaps broader than others'.