Using Supernotes 3.0.
Uploaded a screencast here: https://streamable.com/76oi1z
Thanks so much for the bug report @MaxLinWorm, the screen recording is very helpful.
I’ve been able to replicate this on our end, we will push a fix for this soon. In the meantime please use the Modify card prompt in the Universal Search or Add Parent coupler while editing a card to add cards inside each other.