You can see this behaviour when you try to export your Report for example to PDF.
Some of your tables can be spited between two pages even if the “Keep together on one page if possible” property of table is checked.
In some cases this property can be ignored and has no influence on that how your report will looks like. To solve this problem you have to check if the “Keep together” property for grouping is set to True:
If this still doesn’t solve this problem, you can also check this property for each row in table. Change the visibility of Rows/Column Groups to “Advance mode” and set the “Keep together option for every field.