I have checked QML file structure in QGIS 3.16. And your QML file is not corrects.
This line
<rule label="Urgency Classification" key="{97cbfa40-fc6d-4947-b8e0-e0249feef0ef}">
not have filter and symbol number. Filter and symbol number are in the next line
<rule label="UC NULL" symbol="1" filter=" "Priorisierung" IS NULL OR "Priorisierung" = -1" key="{4aa49c55-6372-493c-a878-bfa6d9556c66}"/>
You have to try to riarange QML FILE, puting filter and label in the main rule tag so (check only that filter is corrects):
<rules key="{63dbbb5f-7045-4662-840c-38d549e97198}">
<rule symbol="0" key="{e678f11f-2cbe-4513-99fe-31c8ecdc4580}"/>
<rule label="Urgency Classification" key="{97cbfa40-fc6d-4947-b8e0-e0249feef0ef}" symbol="1" filter=" "Priorisierung" IS NULL OR "Priorisierung" = -1" />
<rule label="UC-" symbol="2" filter=" "Priorisierung" = 0" key="{dae3145c-3045-4eb3-91bf-49c024514dbc}"/>
<rule label="UC1" symbol="3" filter=" "Priorisierung" = 1" key="{46e4ced4-89c7-4fce-a1b2-797a89ce52fa}"/>
<rule label="UC2" symbol="4" filter=" "Priorisierung" = 2" key="{52485093-9bf9-401c-8a7a-4c1cce6448fe}"/>
<rule label="UC3" symbol="5" filter=" "Priorisierung" = 3" key="{3d9ca004-ea0a-477a-8d7f-49e098253ee8}"/>
</rule>
From which QGIS version derives your QML file?
If you don't want rewrite QML File you can riorganize your stile in QGIS, Putting sub-class under your main class