"I'm populating this dropdown from another list filtered by another dropdown value.", I'm not quite sure how did you achieve this. Could you please share more detailed configuration? Including how did you set the another dropdown value. Maybe some screeshots could help understand better.
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Community Support Team _ Mona Li If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.
1. Department Field (Sharepoint single line of text) - in the powerapps form, i added a custom dropdown control in the datacard (Department_dropdown).
2. in the Department_dropdown's "Items" property, i set it to Filter(<Departments Datasource>, Title=<dropdown2.selected.value>).Department. This will populate the department with items filtered by the dropdown2 selected.
3. in the Department_dropdown's "Default" property, i set it to If (DisplayMode= DisplayMode.Edit, ThisItem.Department, Parent.Default)
As you can see in the screnshots, the Department value should be PCG and not FCG. But the weird thing is that it doesn't happen all the time. it seems like the filtering was able to run but the setting of the default value didn't proceed.
You can test this by creating multiple items, then try opening items quickly.