It's almost certainly A/B test. Essentially everyone does it.

You can even sometimes see when a list of videos on YT first loads and a given video has one title, and it suddenly switches to another without you reloading the page.

Have you noticed that has become more common recently? Because I have. It seems over the past 3/4 months, reload and see a new title happens pretty much daily to me (it also seems to work because sometimes it causes me to click the video I didn't click with the old title).

On several channels I watch, I've noticed the top comment is always along the lines of "who was here when the video title was XYZ" because you know it's going to change.