From https://www.mediawiki.org/wiki/Help_talk:Magic_words#Formatnum_outputs_extraneous_spacing
manglePreprocessorResponse puts categories on their own line. For example,
> echo "{{formatnum:22,000,000}}" | php bin/parse.php
<p data-parsoid='{"dsr":[0,24,0,0]}'><span about="#mwt1" typeof="mw:Transclusion" data-parsoid='{"pi":[[]],"dsr":[0,24,null,null]}' data-mw='{"parts":[{"template":{"target":{"wt":"formatnum:22,000,000","function":"formatnum"},"params":{},"i":0}}]}'>22,000,000
</span><link rel="mw:PageProp/Category" href="./Category:Pages_with_non-numeric_formatnum_arguments" about="#mwt1" data-parsoid='{"stx":"simple","a":{"href":"./Category:Pages_with_non-numeric_formatnum_arguments"},"sa":{"href":"Category:Pages_with_non-numeric_formatnum_arguments"}}'/></p>which seems to cause some confusion for visual editors. It also means we need the code in PP/Handles/LiFixups.php
Remind me again why this is so?