Hi Marion!
Thank you for sharing the description and screenshots of the behavior you're noticing with your Storyline 360 course. Sorry that you've run into this snag where the tail of your caption bubble is being obstructed!
This is an issue our team is looking into, specific to shadow and glow effects using captions and callouts. I've added this conversation to the report as we track user impact and will update this discussion as soon as we have any news to share.
In the meantime, I've seen other users work around this by rotating the shape by 90°.
You're in the right place to stay up to date on any new information, and here's a closer look at how we tackle bugs!