I figured it out. it is because of this code in the extension.

public static function setConditionalRights() {
		global $wgGroupPermissions, $wgFlaggedRevsProtection;
		if ( $wgFlaggedRevsProtection ) {
			// XXX: Removes sp:ListGroupRights cruft
			if ( isset( $wgGroupPermissions['editor'] ) ) {
				unset( $wgGroupPermissions['editor']['unreviewedpages'] );
			}
			if ( isset( $wgGroupPermissions['reviewer'] ) ) {
				unset( $wgGroupPermissions['reviewer']['unreviewedpages'] );
				unset( $wgGroupPermissions['reviewer']['validate'] );
			}
		}
	}

This automatically removes the unreviewedpages and validate rights from editor and reviewer when $wgFlaggedRevsProtection is true.
I don't know why it does this, but I would suggest setting $wgFlaggedRevsProtection to false is it seams to disable some functions.

Accualy , it seems to be becuese $wgFlaggedRevsProtection is considered some sort of "simple mode" or "simple config" of the extension.
I suggest setting it false to enable full functionality of the Flagged Revs extension.

After unsetting that config you can remove the editor and reviewer rights grants from the config as they are the same as the extension's defaults.

TBH, I'm not sure we really want/need to change it. Unless there is some sort of need for testing that particular toolset.

This is a non-default option that makes flagged revs work only as a protection system. It is not a common setup and disables a lot of functionality. Would it not be best to allow testing with the most common/default configuration. We already have a protection system that works just fine, with this enabled I think that there is almost no point to having Flagged Revs installed at all. Please disable this so we can test the full/default functionality of the extension.