First jacket page gets deleted on conversion

[Geert Cuypers]

I like to add a 'jacket page' with the 'polish books' command so I won't have to look up the book description again later. However, if I convert that book after adding the jacket page, it disappears again. If I merge several books (all with their own jacket pages) and then convert, usually only the 1st jacket page seems to disappear.

I consider this a bug.
After all, I go through the effort of making the jacket pages, conversion should not get rid of it again.

Preferences => Common options => 'structure detection' has an option to add an extra jacket page.
I leave that one unchecked, to not duplicate the already present jacket page.
NOT checking that box to ADD one should imo not mean that conversion will remove an existing jacket page, but that seems to be what is happening.

I figured out a bypass by editing the file before conversion and deleting the metadata name =jacket" line.
Still, that is again an extra steps that should not have to be done.

TIA.

Calibre 5.8.1 on Win10.