There is a clear explanation for that.
The person who first hit the crystal was the Badon Bow user. He did it with the thetford cape when he hit you right after using the badon E.
In this moment the crystal, which is also considered a monster, 'aggroed him'. Since he is too far from the crystal, it 'reseted'. After reseting monsters tend to have some instants of invulnerability, which was the case in the video.
Everything as intended, not a bug.
There can be some talk about how it shouldn't be like that (maybe increasing significantly the range at which crystals reset would be a way).
But definitively not a bug.Corrupted PvP Vídeos: youtube.com/channel/UCGXW1-9uTPX5zGIpxRuA1rQ
Thank you for the quick reply and for clearing that up.