We had the same issue on customer site with Window 10 1903 (provsioning and mounting system same Master) and after 2 days of searching, We found the issue.
Its a new GPO Blocking Untrusted Fonts. Disable Blocking of untrusted Fonts and all went good. Customer has much more Apps in Appstacks installing untrusted Fonts, now all issues with Fonts gone.
Be aware that this is a Computer GPO. If u use Instant Clones, u should Push new Image, to have a Replica and Parent Maschine with that GPO applied. Otherwise new created VMs from Old Parent dont have that GPO applied and u wouldnt see the correct toolbar. In Appvolumes "svservice.log" u will see that all registering and installing fonts tasks are failed.
No nwe Provisioning of Appstack needed after GPO is disabled. This will save many time :-)
- Configuration\Administrative Templates\System\Mitigation Options\Untrusted Font Blocking = Disabled
Computerkonfiguration\Administrative Vorlagen\System\Ausgleichoptionen\Blockieren nicht vertrauenswürdiger Schriftarten = Deaktiviert