Hello,
OOXML specifications require that at least one content must exist between tables. It's a MS Word requirement and it can't be avoided; if you don't add at least a paragraph between two existing tables a corrupted message may appear when the document is opened. That's the reason because phpdocx adds that empty paragraph (<w:p/>) automatically if no other content exists between two tables.
You can add a paragraph with the minimum space between tables. Please check the documentation available on https://www.phpdocx.com/documentation/snippets/tables-without-spacing-between-them. The same can be done using HTML methods.
Regards.