Credits to Thomas Rebele for discovering the issue.
When SemiJoinRule (both CoreRules.JOIN_TO_SEMI_JOIN and CoreRules.PROJECT_TO_SEMI_JOIN) matches an ANTI join, it fails with the following error:
The problem is that the rule config only forbids RIGHT and FULL joins, and lets ANTI go through. Later when the rule is actually executed, joins with type ANTI cannot be handled, hence the AssertionError.
The rule config must be adapted to forbid also ANTI joins.