Your sample it works.
And you ask for a sample from my side, but as you can imagine, the project that my company have build is big, and i cannot provide a simple sample, the only way was to share a huge sample, that would make me a lot of effort / time...
I've one main page (Main.razor)
I've one component with SfDialog with multiple DropDowns. (DialogCreateUser.razor)
On the mainPage I embed the Component, and when I ask for the component "DialogCreateUser" to Show() it shows the Dialog without any issue, and the DropDowns are filled with data.
If the SfDialog isModal=True, when I try to open a DropDown the <items...> are not visible .
if the SfDialog isModal=False, the DropDown show all the <items....> without any issue .
If I remove all the other HTML on the Main.razor, isModal=True start working not creating issues on Dropdowns, but if i start adding some HTML the DropDowns stop showing results.
And yes, i've check if there is some CSS that is breaking, and its totally clean.
I can share the link in private and the User and Pwd for you to check in your side.
On tablets and Pcs is working without any issue, on Smart phones is not working .