When a player is created without an id on the embed code, Video.js automatically assigns it one based on an auto-incrementing number (a.k.a. a GUID). For the longest time, this has happened to result in the default id of the first player being vjs_video_3.
It was never intended for users to rely on this value being consistent, but users do strange and inadvisable things.
PR #6103 had an unintended side effect in that it changed the default id to vjs_video_2, which we worry could affect some users of Video.js.