Excel only allows up to 255 characters in Validity's list length - so we need to document that in help
Something like "Note: the length of list can't be longer than 255 characters if saved to Excel formats, and will be truncated on export" in "List" entry.
Seth Chaiklin committed a patch related to this issue.
It has been pushed to "master":
tdf#137277 add note about truncated lists in Excel export
The updated version is now online at: https://help.libreoffice.org/7.1/en-US/text/scalc/01/12120100.html