In my custom theme I created a custom template for one of the default c5 blocktypes and tried to use it for a block inside a global area. But I experienced that I cannot use custom templates neither in global areas nor in stacks (the respective selection field is empty there). Although I'm working with c5 more than 10 years and created many custom templates, I wasn't aware of this behaviour, but I also don't remember if I ever tried it. Is this a feature? Or is it likely to be a problem with my installation?? Has anyone experiences with this?

concrete5 version 8.5.2
MySQL version 5.6.42-log
Apache/2.4.41 (Unix)
PHP version 7.2.27