I havent reported it to AnswerHub as I thought it might be covered by the open ticket already…
Here is a short demonstration of the effect. I can reliably reproduce this behavior.
At first glance I was able to sort of create a material instance struct member as well, but then again not. The error is somewhat peculiar.
I made a short video demonstrating the issue (before writing a whole wall of text).
It shows you a couple of crashes. Sometimes when the member is added, sometimes when a value is assigned. sometimes just by closing and reopening the struct editor panel.
If you need any more info (like the project files, etc.) let me know
Thanks for looking into this.