Unfortunately the GUI for Choice parameters is not properly handling that character as it is used as the separator between different choice values so currently setting %Y%m%d will become three choices: Y, m and d. I have raised issue FMEDESKTOP-9147 to request this is fixed.
As a workaround I would suggest using a different symbol and then using a ParameterFetcher > StringReplacer in your translation to rectify this.
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