Hello once again @kwebbink
Thank you for the email!
Taking a closer look I was able to individuate the issue, and its due to this behaviour:
We always prioritise “Single line string” fields as titles for blocks, over multi line strings. The “class” field inside your block is a single line string, and the one above it is a multi line string, so the “class” field will always be the title for this block.
I see how being able to select a title field manually for blocks could be helpful in this case, could you perhaps open a feature request for this so we can track it?