This doesn't seem to fix the original issue. You can try it with the following:

editor.asDelta().setValue(
                "[{\"attributes\":{\"bold\":true},\"insert\":\"Line 1\"},{\"insert\":\"\\n" + //
                                "\\tIndent 1\\n" + //
                                "\\t\\tIndent 2\\n" + //
                                "Last line with\\textra         spaces and a \\ttab\\n" + //
                                "\\n" + //
                                "\"}] ");

The above will still produce:

In fact, the result is the same even if you return the given html as such from the sanitize method.

I need to dig this deeper. It may be that the fix needs changes in web component as well. Or if the white spaces are stripped also in Quill. If the later, we are stuck. If it is not the trouble maker, i.e. issue in our code, this Java side change is needed anyway, but if it is Quill related, this change is meaningless.

Did some more investigation. This is actually a proper fix for the Flow RichTextEditor. To fully cover the original issue, another fix to the Web Component is also required. I'm working on that now. Once the Web Component with the fix is available, we can merge this change and close the issue.