There's an additional backdrop for the generic multifield dialogs. It is being injected into the DOM and is getting displayed only if the original backdrop is hidden.
This custom backdrop was being removed from the DOM once the last generic multifield dialog has been closed, with the presumption that the original backdrop is getting displayed. But for some reason this was not the case.
The fix is to only remove the custom backdrop when the final dialog is being closed. This way it's ensured that there is at least one visible backdrop.
There's an additional backdrop for the generic multifield dialogs. It is being injected into the DOM and is getting displayed only if the original backdrop is hidden. This custom backdrop was being removed from the DOM once the last generic multifield dialog has been closed, with the presumption that the original backdrop is getting displayed. But for some reason this was not the case.
The fix is to only remove the custom backdrop when the final dialog is being closed. This way it's ensured that there is at least one visible backdrop.