As you've opened a support ticket on the same topic, I'm going to quote the response from my colleague Rick:
"Thank you for reporting this issue to us. I was able to easily reproduce the issue from your description.
Justified alignment producing broken spaces with underline styles is a rather old issue in some rendering engines. The Chrome Browser used to suffer from it as well, as you can see in the resource below.
In my tests, I found that the issue is present in the report designer "preview" pane, but not in the "HTML Preview" pane or any tested browser. I also verified that the issue isn't present in exporting PDFs.
Are you able to replicate the behavior outside of the "preview" pane of the report designer? It's possible to use non-breaking style spaces, but if the issue does not present itself outside of the preview area I would recommend disregarding it."
We can also continue the discussion in the support ticket #1161304.
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