Just to confirm that indeed Libreoffice 5.2.1-1 in testing also has the
issue. Sorry I don't remember when it appeared, but as Simon John said,
it was some time between late August and the first half of September.
I also want to add one symptom that I have (and maybe Simon John can
confirm?): dialogs are extremely slow to open. One example is the
paragraph style dialog in Libreoffice writer.
Thanks Simon John for the workaround: indeed uninstalling
libreoffice-gtk3 solves the issue (UI style becomes ugly, but it works!)