I know that a refund will affect your “total spent today” amount, but not sure about how it affects the summary. One workaround, for the case where you get a full refund for a transaction is to exclude the original transaction from summary, which isn’t the case here unfortunately, where you have a partial refund.
This is the single most annoying feature of Monzo and why, when I can face it, and despite being a Monzo investor, I’m going to move to Starling. Starling net off any credits put into a category from the total spend. There are so many examples when the way Monzo do it is really annoying. Eg, Taking something back to the shop. You might not be able to exclude the original transaction as it might have included other things. Also, I regularly buy cinema tickets for myself and a friend. She pays me back but I can’t offset that against my Entertainment category. I could go on. It drives me mad. I can’t think of any good reason why you wouldn’t want to net it off.