When creating a DockPane with a BurgerButton, the first time you click on the BurgerButton, the context menu appears underneath the button. However, if you then click on an item inside the contextual menu, with any subsequent opening of the context menu, the context menu will appear to the left of the button instead of below.
This is reproducible in the CustomCatalog Esri Community Pro SDK sample:
https://github.com/Esri/arcgis-pro-sdk-community-samples/tree/master/Framework/CustomCatalog
Thanks for reporting this. I see this issue also. I will notify the Development team.