I made a bug report about this here: https://scratch.mit.edu/discuss/topic/829648/ But the bottom line is this: The vector costumes in many projects are corrupting, and it is getting worse every day. Even if all your projects are fine right now, they won't necessarily stay that way. If you have any projects with vector costumes that you care about keeping, download them now, as this bug does not affect the offline Scratch 3 editor. How to download your project: In the bar at the top of the project editor, click on file, under file select "save to your computer".
