I believe this is going to be a timing issue sort of, the first click is setting the variables, but the subsequent evaluation in the expression is not occurring after the setting of the variables, that is likely occurring before the variables etc. would be my best hunch. I do not believe there is any means to control things here either unless you can somehow do things all in a single expression if that makes sense as then you should be able to control the order of things. Hopefully makes a little sense.
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