Thanks for raising this!
I've noticed it too, it won't do it when you first open the page:
But it will break that way once you refresh the page:
I'll submit a bug report for it if one doesn't exist already.
To get around this in the meantime, reopen the page in a new tab and it should be ok / you should be able to subscribe.
Edit: It has already been reported here, hopefully it will be fixed soon: https://github.com/LemmyNet/lemmy-ui/issues/1581
People are also mentioning this workaround: The way to resolve is to click on any link on that community and the Subscribe/Block buttons show up again.