I think this is only happening since Jenkins core 2.238 and thus I extremely naively suspect an unfortunate side effect from
JENKINS-61973 (Félix Queiruga Balado), but of course I might be wrong...
See screenshot: Actually the layout jumps – initially the build display names are shown full, and then (via JS?) the first one is wrapped, while all the others are just truncated:
Oops , when no build is running for the pipeline job, then it looks OK: Unable to embed resource: image-2020-05-29-09-48-48-840.png of type image/png
If there are less stages in a pipeline the build display names are shown in full the whole time, but the one for the currently running build is wrapped: