Maybe you need to tweak your copy of oXygen to use a fixed-space font instead of a font with proportional spacing.

Also, oXygen has an option for preserving text in element content as is
(Options > Editor > XML > Formatter) that applies when you format an
XML document in the editor. If this option isn't checked (it wasn't on
my copy of 13.1 and I had never touched it), oXygen collapses whitespace.

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