Can you please add more details what you tried?

In the past few years, we had not set the threshold for the low priority issue. But due to a reorganisation, we now wanted to enable email notification for the developers which checked in also for these low-priority issues of Findbugs.
Therefore we set the threshold such that every new low priority issue will result in an unstable build. So far so good.
But sometimes, these low priority issues are just ok and we do not want the build be unstable forever.
We would like that developers gain ONE email notification when the warning appeared the first time and not after every subsequent build.

Therefore I thought that unchecking the setting "Only use stable builds as reference" should result in exactly that. Every subsequent build should calculate the number of new warnings from the last build (also from an unstable) and not the last stable one.

This option only applies to builds that are unstable due to test failures. It is not influenced by the unstable result of the findbugs plug-in. A reference build is always the build that is successful (with respect to the findbugs plugin).
If a warning is acceptable then you should mark it with @SuppressWarnings annotation.

Or is JENKINS-13458 something you are actually looking for?

JENKINS-13458 is kind of what I was looking for, if specific to FindBugs (means setting the flag for findbugs does not set it for all others too).
I voted for it.

Maybe the tooltip on the flag "Only use stable builds as reference" could better reflect the current behavior.

Ok, I changed the title and description.

sounds great, thanks.