Hi James,
I did a test in my own account, doing my best to replicate what you are suggesting. It appears to be working for me, but I would like to confirm what I have done is similar to what you are attempting. I created a new screen based off my Customer table, then added a Customer grid, I included fields Sales Rep, RecordID, and Customer. Below my grid, I added the "Sales Rep" dropdown field, whenever the dropdown is changed, I have a filter view set to filter the grid depending on which sales rep was selected. I have attached some screenshots below:
Unfiltered Grid View:
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Filtered Grid - From PC (before making this screen "Mobile Optimized"):
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Filtered Grid - From Mobile Phone (After making screen "Mobile Optimized") - Click link to open image in a new Tab
Screenshot of the actual Filter for "SalesRep":
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Keep in mind that if you do make your screen "Mobile Optimized" it might not appear correctly in a regular Computer Web Browser, so double check how it looks on a smartphone or tablet for proper testing. Please let me know if this was helpful, if you need more info or if I missunderstood what you are attempting please correct me here.
-Ben