This is an issue that was initially reported on a Site and I could not replicate it when testing in any browser, until I thought to test on a separate monitor (not my laptop screen). When testing on the monitor was able to replicate in Chrome & Edge through the Site, through the top-level parent interface, and through the interface that contained the dropdown (although for the customers instead of a refresh, it was causing the entire page to turn grey). Can only replicate the issue on my laptop screen through the interface, not through the Site. I have reached a resolution for the issue (updating/deleting rows in the database or logic on local!choices to replace nulls with "Unknown") but sharing since I was not able to find anything when I had searched online. Believe the issue was caused by a user deleting some of the data for a couple rows before using the "Delete" icon/button that had been set up.
The case was the query pulling in the data did not have any filters & there were couple similar fields in the database: code, desc, name. The queried data was sorting on the code (always not null) & using the desc (couple nulls) as the Choice Labels (IDs as the Choice Values, all unique integers). Below is some code that can be copy-pasted into a test rule if anyone would like to see what the issue looks like, just need to do the testing on a separate monitor (not your laptop screen).
a!localVariables( local!values: enumerate(105), local!choices: touniformstring({enumerate(15), "", enumerate(89)}), local!selection: null, a!sectionLayout( contents: a!dropdownField( label: "Dropdown", placeholder: "-- Select a Value --", choiceLabels: local!choices, choiceValues: local!values, value: local!selection, saveInto: local!selection ) ) )
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What I understood is that the choice values are the Ids that are not null but the choice labels are non id fields that are nullable, here you need to put a filter on the query to return only that data which will not have nulls in the field used in the choice labels.
This was just a test interface to show/explain a bug. The actual interface had ID fields from a database as the Value & Description fields from a database as the Choices. A description had not been added to 3 of the 100+ rows. Having a dropdown with 100+ choices but 1 or more of them being blank causes this issue (Stefan replicated it & added a video).
I had forgotten to mention in the post originally, but for the users, this issue was causing the entire page to turn into a gray screen (needing to manually refresh the web-page to fix). But for me it was looking like a page refresh, although selected values on the page were being retained for me.