Hey Felvol,
I was able to reproduce issue you’ve described, and entered a bug report in our database for our developers to look at (TTP# 348329). I will let you know when I see a fix for this.
In meantime, and for any other users experiencing this problem, opening Color Picker in node and saving default value or using a Linear Color variable and plugging that into Rect Color input will ensure rect is drawn.
Thanks for pointing this out to us!