Bug#1035899: (no subject)
Updating xwayland to version 2:21.1.3-1 removes this particular bug (and introduces others). So the package containing the program that issues the call with -initfd must depend on xwayland version ≥ 2:21.1.3-1. I have no idea which program it is or whether this has been already done upstream.
Bug#1035899: (no subject)
I noticed now that I had unintentionally omitted the name of a package in my version list in my original post. It should read: “Here are the versions of some relevant installed packages: xorg 1:7.7+23, […].”