1. Two titleblocks are created with 2 different names. The border is a another block inside each titleblock.
The 8.5x11 border block in the 8.5x11 titleblock has the same name (TB_BORDER) as the 11x17 border block (TB_BORDER) in the 11x17 titleblock.

2. The 11x17 title block looks fine. There is no sign of other block entities in the drawing. We don't see an addtional 8.5x11 border in this drawing.

4. To fix. Rename the internal TB_BORDER block. You can use the RENAME command without exploding the blocks.
5. If you need a fast temporary fix, attach the TitleBlock as an XREF .
Best Practices: Keep block names distinctive, descriptive and different for each entity in your block library. This includes blocks you are using to build other blocks.