Diagram attached. 

When there are sub reports and KeepTogether would require a dataset to use multiple pages the report renderer fails to honor page breaks. Rather than use the new page it should be using to present the page, it bleeds onto the prior page which has a page break set. 

See Ticket #1144729 for all the relevant information regarding the version, setup etc. and attached examples. I was directed here to the feedback portal so your staff has led me to believe this is an existing issue with the current iterations of the report project.

Additionally me original forum post below.
https://www.telerik.com/forums/keeptogether-not-honoring-page-breaks