I found a bug on my XFCE and some other can't repeat the bug, namely https://askubuntu.com/questions/495947/a-logo-disappear-in-xubuntu . How should I report it to the developers if those can't repeat is as well?
The fact that they can't reproduce it shouldn't bother you. Since you can, you report the bug as is, telling exactly what you did to reproduce it, and what is your system.
Either there is an actual bug affecting many people, in which case there are chances that either somebody else will reproduce and confirm it, or developers will look at the code and find that indeed, there is a problem with it,
Or the bug affects only you or a very small amount of users (for example users who installed a specific version of a specific software product while having a specific driver for a specific device and a specific system file missing), in which case, there are risks it will never be solved.
Reading closer your linked question, it seems that:
The problem is too minor: it doesn't look like this problem impairs your ability to use the OS properly. I'm pretty sure there are many currently opened bugs which will have a higher priority than this one.
The problem might be related to your GPU, in which case it affects only people using the same GPU or type of GPU, which highly reduces the number of potentially affected users, thus reducing the chances for this bug to be solved.
But still, create a bug report, explain what you do to reproduce the problem, just like you did in your linked question, and specify the version of the OS, the drivers you have installed, etc.
Note: if you can actually reproduce a bug on a virtual machine, this could help a lot: you will then be able, for example, to share the virtual machine with the developers, in order for them to actually see the issue.