Closed the page and re-opened my program but it is incomplete.
Loading stops when it reaches the custom block

What it means is the custom block file needs to be loaded before loading your program file.

I suggest this is added to the help here: https://docs.m5stack.com/en/uiflow/blockly_custom?id=using-program-block

The User Manager link in the custom block section goes to a dead link: https://docs.m5stack.com/en/related_documents/blockly_custom

It should go to https://docs.m5stack.com/en/uiflow/blockly_custom