Does it prevent you from uploading the post? AFAIK it always makes you select which community when creating a new post, which would be normal functionality, not a bug.

Could you provide some more detail, maybe a screenshot illustrating the problem?

EDIT TO ADD: Are you selecting "create a post" from your front page, or are you navigating to your community first? If it's the former, then of course you will be asked to select the community; if it's the latter, then it should fill in your community by default (while still giving you the option to change it).