"Operation not permitted" in Singularity containers

I'm trying to make use of fuse-overlayfs inside a Singularity container (based on CentOS 7) by using its new `--fusemount `option. Ideally I want to use this to make a writable overlay on top of a read-only CVMFS mount, but basically I'm just trying to do something like this:
```
singularity shell -B /overlay --fusemount "container:fuse-overlayfs -o lowerdir=/overlay/lower -o upperdir=/overlay/upper -o workdir=/overlay/work /merged" fuse-overlay.sif
```
With recent version of fuse-overlayfs this seems to work partly: I can do some operations, like making new files, but some stuff does not work. For instance, making directories with `mkdir -p` (and sometimes even just cd'ing to the merged mountpoint) usually results in an `Operation not permitted`, e.g.:
```
Singularity> mkdir -p /merged/my/new/dir
mkdir: cannot create directory '/merged/my/new': Operation not permitted
```

Today I more or less accidentally found out that with a Debian 10 container everything does work fine. The default version in the Debian repo is quite old though, it installs fuse-overlayfs 0.3. Testing a bit more with various manually compiled versions, I found out that for both CentOS and Debian containers everything seems to work fine with versions up to and including 0.4.1, but with version 0.5 and newer I consistently get these errors.

Do you have any idea what could cause these `Operation not permitted` errors?