I just discovered that recent macOS 10.13 ignores requests to set the EOF of the resource fork. When I wrote the test a few years ago, testing with 10.10 at the time, behaviour was what it was and I then modelled the tests and vfs_fruit after this.
The observed behaviour is in fact a bug in macOS and will be fixed. This was confirmed by Apple in a private mail exchange.
Therefor closing this bug filed against Samba as invalid.