I tested and reproduced your problem in my own environment.
According to my research, I have not seen an explanation of this phenomenon in any official documentation.
I also tested other column formats, this problem occurs only when a custom button that calls flow after a search occurs.
It does not occur if we call email or other behavior.
This looks more like a bug or error.
I'm sorry, but so far I haven't found an alternative workaround.
I recommend that you submit feedback below this official document and look forward to subsequent explanations and solutions.
If the response is helpful, please click "Accept Answer" and upvote it.