Again, this is a long standing known issue.
I could talk for a long time on why it only affects the ticket inspector handhelds, how TfL contactless works at a high level and the technical choices TfL made to get here with this issue. The short answer is that the handheld readers will try to authorise an offline transaction for the amount of the maximum fare and then either drop it or commit it to the payment networks later when the journey history for the card is all collected and calculated.
Monzo cards are, at best, weird about offline and will almost always reject any attempts at an offline contactless transaction, forcing the terminals to go online for the instant notifications and card freezing feature to work. This rejection causes the handheld readers to error out and fail your inspection.
The good ticket inspectors will just ask if it’s something like a foreign card and let you go, others want a bit more explanation (you should use the line from the known issues post, trying to explain the technical side gets you in even more trouble 
).