Date in file name now fails intermittetly [closed]

This command in a script which runs repeatedly in both test and a production environment and has been running for years now sometimes fails to create a name for the file. Just leaving it .xml

fextension=xml
bigFname=`date +"%Y-%m-%d-%H-%M-%S"`"."${fExtension}

So when I test it in a small script I get for example this:

2026-07-02-16-08-51.xml

Why would this now sometimes fail? So far it is only happening in some test environments.