The following options are set:

Status Threshold (Total), All priorities: 0
Compute new warnings (based on reference build): true
Use delta for new warnings: false
Only use stable builds as reference: false

Expected behavior:

The number of new warnings is always calculated with the previous build as a reference.

Observed behavior:

The number of new warnings is calculated with the last stable build as a reference.

We have other jobs without that threshold setting which correctly calculate the new warnings with the previous build as a reference independent of it's status.
I suspect that the last stable build is selected as reference because of the threshold setting. If findbugs is the cause of an unstable build it seems to be rejected as a reference even though the "only use stable builds" option is not set.