Why does vim ignore commentstring when folding (foldmethod=marker)

Question

I am editing a HTML document (filetype=html) in vim. foldmarker is set to {,}, foldmethod to marker and commentstring to <!--%s-->.

When I visually select a region and use zf to create a folding, vim inserts a { at the beginning and a } at the end of the region. However, with my settings, I expected vim to insert <!--{--> and <!--}-->, respectively.

Why is vim ignoring commentstring when folding?

Answer 1

This answer probably should be deleted. Answering here to show it @ChristianBrabandt.

It is kind of strange but for some text it works, for the other it doesn't:

And if you change the first paragraph to:

blablabla a
lkjsdlfkj
lksjdflksdj
lksjdflksja

Then it will not work for the first row. Looks like if there are more than 1 word on the line it doesn't insert commentstring.

vim version:

VIM - Vi IMproved 8.1 (2018 May 18, compiled Dec  5 2018 23:02:23)
MS-Windows 64-bit GUI version with OLE support
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