Sorry that this happened. We still have a long way to go to improve our billing system. We have allocated more people to it since a few months and are making much faster progress. But it isn’t we’re it should be at the moment.
Thanks. No worries, these things happen. It's just a little alarming to see messages like "we've downgraded you to the free plan". Even a bit of help text in the banner, linking to a page on situations under which the message might be raised / not be 100% accurate, might ease the experience.