We have a pipeline with a single pipeline-level post block to do a workspace cleanup upon success:
The rest of the pipeline has a variety of sequential and parallel stages – I can supply some specifics if they might be significant. However, every stage as rendered in the Blue Ocean UI shows a final step of "true – Delete workspace when build is done", complete with the corresponding console log from this:
The console log from the classic UI confirms that this step is only occurring once, at the very end (as expected).
JENKINS-47251 as a similar issue but resolved, and perhaps specific to the use of a script block.