@lanodan_wtch The ConScript Registry uses the private use area codepoints. However those codepoints can be used by anyone for any purpose. So you may have fonts with an higher priority displaying symbols used for something else instead of the one you want to see.
You should force the font when displaying those scripts.

About Cirth, there is a proposal made twenty years ago to include them in Unicode, along with Tengwar. The proposal is still in the waiting list.