Earlier quoted context omitted.
Perhaps some old integration code from 10+ years ago that was written to spit out XLS rather than XLSX e.g. some old versions of Crystal Reports can only do XLS. There must be loads old systems around that can only do XLS
XLSX files have been around for ... nearly 13-years... Ugh - I still deal with clients who are creating brand new XLS and DOC files and expecting to have all the latest features (co-authoring when hosted in 365/SharePoint Online/OneDrive/Teams)...
Even the current version of Excel still inexplicably auto-converts long numerical strings to Scientific Notation.