I've realised that this is actually a feature rather than a bug.
The bar is rendered hollow to indicate a market gap - ie there was no quote on the book within that range.
Never seen this on any other platform, which is why I didn't immediately grok it. But once you understand what it signifies it's pretty useful for paper trading.
So kudos to the team for inventing this
But it would have been easier to understand if they had replied to the original post
G