The block-container div has the bottom left corner radius mucked a bit if there is no footer block (like a submit button). An example is on the Account Upgrades page.
Have you noticed this anywhere else? Right now it looks like it's caused by the massively non-standard HTML in use on the account upgrades page. If this is the only affected page, I'd be inclined to just junk the HTML used there and start again so it makes use of existing CSS.
It's the only place that I've personally noticed it, but I didn't do an extensive search into finding other instances, but I did skim some basic pages that I was curious if it was an issue on (like all the account pages), but didn't see it anywhere else.
Incidentally, I did notice it just yesterday on something I did, but was easily fixed by moving the <xf:form> tag I was using to outside block-container. I didn't go mucking with the account upgrade page, but maybe it's the same thing? Although if that's the case, it's a little tricker fix since there are multiple <xf:form> in the block-body.
The lazy fix is to just add a footer, but probably not the most ideal solution since maybe you don't actually want a footer everywhere.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.0 Beta 4).
Change log:
Fix cosmetic issue on border radius with weird HTML configurations at the bottom of blocks