This is of course something going on all the time. I don't decide to take notes of the traffic on a gas mine on pure random. I have in the past seen 6000 spice reserved by my ally to game's end. That is annoying, because even if you got the source, there is not unlimited amounts.
My chief suspect is that something interrupts the transport. An attack, a retrofit, a retire order, other amnesia. If attacks resets it, it could explain this happening quite often. The "escape" should be a new, temporary order, not a replacement for the old. And if it replaces the old, the ship should release its reserve.
I did not see any production of krypton and argon either, but I think that is about the base working at its storage limit. I think it works to try balance the cargo among its products, and either don't produce argon or jettison it.
What it is not possible to know, is if there suddenly is a huge argon demand, that the private sector could figure out "hey, we got 10k sitting reserved over here, let's get some".
To test this a work, I would suggest a quiet game where not much happen. At least not military ships hunting for fuel. It helps if your ships use either a mix of fuels, or hydrogen. Build a resupply ship, and park it at a pure caslon source (or at least one with no hydrogen). Wait for it to fill up. Then move it to either a mixed caslon/hydrogen source, or a hydrogen source. I think you will see it reduce it caslon store (which is of course not reserved) to make room for hydrogen. I have had worries around this in the past, and it has worked out fine. It is not necessary to junk your caslon filled resupply ships when you switch to hydrogen, you just need to park them at sources with hydrogen.
About my Argon and Krypton, I don't think these are resources used to a large degree. And they are often supplied with caslon and hydrogen. My empire had over 40 sources for each fuel. And probably 20+ for each of argon and krypton just as a side effect of that. I am not so much puzzled by the lack of traffic in Argon and Krypton, as in the amounts reserved for years on end with no one to pick it up.
This hinges on my understanding of a reserve. I believe a reserve is only placed when construction starts, and when a freighter is on its way. When you start building a small space port on your fresh colony, the materials are reserved. And you see 0 amount 1200 reserved and such. This makes sure no one else can run away with these items if they ever appear on the colony. For fuel, I also think items are reserved once a ship gets "refuel at" order. This is quite easy to test on a resupply ship, btw, since there are no freighters confusing things there. Freighters refuel if possible when they pick up stuff, I don't think any reserves are involved in this. The same as a fleet refueling at retrofit. They happen to grab if anything is around.