AH Formatter / XSL Formatter
Q&A

Your FO is as follows:

<fo:table keep-with-next.within-page="always">
 .......
</fo:table>
<fo:block />
<fo:block>Sample</fo:block>

XSL Formatter generates the area of empty fo:block. Therefore, the next area needed by keep-with-next.within-page="always" is this empty block(<fo:block />). The page might break between <fo:block /> and next <fo:block >.
Empty fo:block generates empty block-area without line-area. The block-progression-dimension value of the area is 0. Therefore, it is never usually seen. In the CSS2 Specification referred to from the W3C Recommendation for XSL-FO, there is the following examples.

In this sample, the line appears by specifying the border property for empty block. This means the empty block generates the area.