There are a few instances where the expression builders preview results will be different than what the actual app shows.
This is due to the fact that the expression builder is something that’s running on the AppSheet editor - while the formula for your app is being run by your app. They have different parsing mechanisms - there was a detailed post about this years ago by Phil I think.
Generally it’s just a matter of reformulating your formulas - thankfully, in AppSheet there is…
If this is the formula you’re working with - the issue most likely is due to the fact that in your tables you’ve got two columns that have the same name:
I would suggest changing the names of these columns so the uniquely identify each from another:
Generally speaking, it’s best to have all the columns in your app have unique names.
Try changing the names and see if that works, your formula is pretty straight-forward, so it doesn’t look like it’s a matter of a complicated formula. I recently went round and round for hours trying to solve a workflow issue - problem was due to identical column names.