Flow Decision activity does not show validation error (Blue/Red Exclamatory icon) besides the activity as we see in other activities in case of any error.
In this case, when building a really big flow, it gets a bit tricky to identify the error if it is in FlowDecision condition. Hence it is a must to have that there.
Screenshot where there is an error in FlowDecision however not reflecting in Designer
Known issue. Decision activity comes from Microsoft, as is.
In case of a big workflow you should manually check the decision boxes if there are no other errors shown.
This can be achieved if you click on Validate button available on the Execute tab. If an activity within the workflow has an error, the activity will be focused