Editoriale Domus Spa
My enviroment: InDesign LTS version 15.1.3 (mandatory) - InCopy LTS version 15.1.3 (mandatory) - Extensis UTC 7.0.10 (mandatory) - clients OS Windows 10/MacOS Mojave- Font family OTF/TTF.

Fonts get activated, then again so that theindesign/incopy document refreshes and detects the fonts but unfortunately this could result in significantly longer loading times for large files and other unexpected performance issues.
The fonts are active for the whole system (reg query "hklm\software\microsoft\windows nt\currentversion\fonts)
and app., Bloc Notes for example.

I have opened tickets with Extensis. Here is the answer after testing in their environment

(Extensis)

Apr 28, 2021, 1:59 PDT

Hi Filippo,

As we currently see it, this is an InDesign issue as our plugin has already activated the requested fonts prior to the document opening fully as indicated by the fonts being present in the Local System registry and being available within other applications like Notepad.

This issue has happened before in previous versions of other Adobe applications and we previously implemented a workaround whereby the documents were opened twice, once in the background so the fonts get activated, then again so that the document refreshes and detects the fonts but unfortunately this could result in significantly longer loading times for large files and other unexpected performance issues.

Ultimately, Adobe needs to address this issue as it should be checking the system registry and updating it's font menu accordingly.