Sometimes (it is random and changes hourly sometimes, it seems) I do not have the option to comment on text that I've selected on a page. Instead, I only have the option to Create a Jira issue. Here are my examples (from the same Space):
No Choice to make inline comment (only Jira issue):

Choice to make inline comment or Jira issue:

I cannot find any mention of this happening anywhere in the Confluence Cloud docs.
Also, just to note: Confluence Cloud is becoming buggier and buggier by the day. I don't know what you guys are doing, but I think that possibly by intending to sandbox your product into oblivion, and by removing all but the absolute bare minimum customization (i.e., like specifying heading level), you may also be killing your code. XML, XHTML, JS, HTML, iFrames, SQL, etc, only work well at so many layers deep . . . eventually it all starts corrupting and eating itself. I get unexpected behavior and errors in Cloud *constantly* now. I have begun creating a journal to show our Atlassian support team.