If you include a page (which also includes two pages using the includeplus macro) using the includeplus macro, text following the include will be displayed in a separate line in the view (in edit everything is displayed as expected, see attached screenshot).
This does work fine in Confluence 5.8 (storage format of 5.9 and 5.8 displayed).
Steps to reproduce:
Create four pages (1, 2, 3, 4)
On page 3 include page 1 and 2 (using includeplus)
On page 4 include page 3 (using includeplus) and add normal text in the same line as the include.
Page is displayed the same way in view and in edit mode (with normal text in the same line as the include)
In view, text is displayed in next line, in edit mode everything looks as expected.
Atlassian changed the behaviour from 5.8. to 5.9. of their HTML Renderer which adds a <div> element around the content sometimes so that the following text is moved to the next line. A user can disable this "dark feature" by setting the dark feature property confluence.wrap.macro.disable on <your_confluence>/admin/darkfeatures.action as described here: https://jira.atlassian.com/browse/CONF-39456