Whenever I want to post feedback on feedback.minecraft.net, I'm unable to use the submit button when I've selected a relevant topic.  To recreate this issue, go to a topic page under the Categories banner, then click "Post new feedback" on that page.  When writing a random idea for testing, select the same Topic as the one whose page you just left.

In this instance, the submit button will stay gray and inactive unless you select a different topic (the screenshots show the sequence of webpages where this happens.  This occurs for any feedback topic).