Movie sort order by duration shows recently added at top

Ever since Kodi 17 / OSMC new skin last year, my Movies category sort order does not work as expected.

I have it set to Duration, shortest to longest. This worked fine before Kodi v17, and still works fine for any movies added up to that point. Movies added since v17 are always placed at the top of the listing.

It’s possible to get the correct order by switching display to a different field, then back to Duration, but next time the Movies listing is shown (eg. go back to Home, then back to Movies), the mixed order is displayed again.

New movies added at the top do not appear to be themselves sorted by duration, they’re seemingly random.

Any ideas on how to get the sort order to work as expected? Perhaps its an anomaly with sorting by duration as I’m sure there would have been other posts / fixes if it were something more common like sorting by name.

Pi 2B (a01041)
Linux osmc 4.14.34-5-osmc #1 SMP PREEMPT Sun Aug 19 19:46:02 UTC 2018 armv7l GNU/Linux