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.
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.
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).
I hate how this has become so normal. It degrades trust when people are fed different versions of the same experience without being informed. How are you supposed to reference something when the title changes without notice? Makes me feel like I’m going crazy sometimes.
Either they changed it for some legal reason, or Youtube allows for A/B testing of multiple titles for engagement.