Here is the first part of the argument:
Me: I got item x from the capsule.
Opp: It's impossible to get item x from the capsule, because such and such.
Me: It is possible. I just got it.
Opp: Oh then it's a bug and needs to be reported as such.
Me: Here is a link from the developers to show that it works as intended.
This part of the argument appears to be complete based on Stephen E. Toulmin's model of argument. A claim has been made that X can be found in the capsule. Data was provided that X was found. Finding X is the warrant that the data justifies the claim. The rebuttal that it was a bug was countered with backing from the link that X could be found which appears to be accepted.
Opp: Oh they must have changed it.
Me: They never changed it. It was always like that.
Opp: Prove me they never change it.
Me: It's your statement. Prove it yourself.
Opp: I agree I'm wrong currently, but it's your statement that they never changed it, so prove it.
In the second part new, opposing claims come from both sides about whether the capsule was changed. What is lacking is data to proceed with the argument.
To obtain this data, the link used in the first part may contain a change history or one could send an email to those managing the link asking for information about the change history, specifically asking: Was there ever a time when X should not have been found in the capsule?
It may turn out that it is not easy to get this data and one side may try to shift the burden of proof to the other. This may or may not be fallacious as Bo Bennett writes:
Making a claim that needs justification, then demanding that the opponent justifies the opposite of the claim. The burden of proof is a legal and philosophical concept with differences in each domain. In everyday debate, the burden of proof typically lies with the person making the claim, but it can also lie with the person denying a well-established fact or theory. Like other non-black and white issues, there are instances where this is clearly fallacious, and those which are not as clear.
Bennett, B. Shifting the Burden of Proof. Retrieved on May 15, 2019 from Logically Fallacious at https://www.logicallyfallacious.com/tools/lp/Bo/LogicalFallacies/222/Shifting-of-the-Burden-of-Proof
Wikipedia contributors. (2019, March 4). Argumentation theory. In Wikipedia, The Free Encyclopedia. Retrieved 14:11, May 15, 2019, from https://en.wikipedia.org/w/index.php?title=Argumentation_theory&oldid=886097165