GAC Issue

Relegations after the squish can't happen until after your first match finishes. Your SR was squished below the threshold before the match started, and winning the match didn't give you enough SR back to get back over the threshold. 

This feels bad and you're not the first person to mention it, but it is normal. It happens to most of us, and those who it hasn't happened to yet will probably experience it eventually. The closer you stay to a line between divisions the more often it'll come up.