If the land of your possession be unclean. — This suggests that they might have built the altar in it to sanctify it. But it would hardly be intelligible unless the altar was, as we supposed, on the eastern side.
Related Commentaries of Joshua 22:19
Joshua 22:19
19 Notwithstanding, if the land of your possession be unclean, then pass ye over unto the land of the possession of the LORD, wherein the LORD'S tabernacle dwelleth, and take possession among us: but rebel not against the LORD, nor rebel against us, in building you an altar beside the altar of the LORD our God.