Animated GIF loops forever instead of once

I created an animated GIF, set the loop to Once, did a legacy export and confirmed it was set to loop once. Upon checking the file after save, it loops continuously.

I also tried setting the loop to Other > 1 (for one play) and that also failed.

Am I doing something wrong?

It's a 5 sec GIF that will be in a PowerPoint slide that will be onscreen for around five minutes. It'll be super distracting for it to continuously loop...

TIA for your help!