How to get the context node in a custom block inside a views list?
When placing the block you have to choose from where the block will get the node value.

Select a node value



Node from URL





Ah! I found my own issue on DO. You also might have to unrequire the context like following:

 *   context = {
 *     "node" = @ContextDefinition(
 *       "entity:node",
 *       label = @Translation("Current Node"),
 *       required = FALSE,
 *     )
 *   }