My hunch as to why the 'Export' does what it does would be due to the BIFF translator component that does the export, this is a Microsoft module, and I suspect it is not quite sure how to handle things in this case. I am not sure which version you are running either, but if something pre 12.20, know that in 12.20, we have updated the BIFF module we use to allow .xlsx now, and that might result in a different outcome as well, but not certain. Claudiu's solution seems to potentially be a good option, hopefully that will work as a workaround on this one.
To help users find verified answers, please do not forget to use the "Accept as Solution" button on any post(s) that helped you resolve your problem or question. I now work a compressed schedule, Tuesday, Wednesday and Thursday, so those will be the days I will reply to any follow-up posts.