Strange optimizer behaviour on optional conditions with functions

explain SELECT *

FROM

rdMPGGeraet g JOIN

rdMPGBezeichnung b ON g.BezeichnungID=b.ID JOIN

rdMPGAdresse h ON (b.Hersteller=h.ID )  LEFT JOIN

rdFunkruf rf ON rf.ID=(IF(g.Funkruf=0,g.StammFunkrufID,g.Funkruf) ) AND rf.OrganisationID IN(42) LEFT JOIN

rdFahrzeug f ON (f.ID=COALESCE(rf.Fahrzeug,rf.Stammfahrzeug,g.FesteinbauFID) AND f.OrganisationID IN(42)) LEFT JOIN

rdWache w ON (w.ID=COALESCE(rf.Wache,f.Wache,g.werkstatt) AND w.OrganisationID IN(42)) 

WHERE

g.OrganisationID IN(42) AND

g.Status="in Betrieb" AND

ISNULL(g.del) 

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	SIMPLE	g	ref	einweisungPfl,BezeichnungID,ClusterID,Status,del	Status	7	const,const,const	1157	Using index condition

1	SIMPLE	rf	eq_ref	PRIMARY,Wache	PRIMARY	2	func	1	Using where

1	SIMPLE	f	eq_ref	PRIMARY,Status	PRIMARY	2	func	1	Using where

1	SIMPLE	w	eq_ref	PRIMARY,Ressource,Shop,Strassensperrung	PRIMARY	2	func	1	Using where

1	SIMPLE	h	ALL	PRIMARY	NULL	NULL	NULL	5180	Using join buffer (flat, BNL join)

1	SIMPLE	b	ref	PRIMARY,Hersteller	Hersteller	3	qmsystems.h.ID	2	Using where

+------+-------------+-------+--------+--------------------------------------------------+---------+---------+---------------------------+------+-----------------------+

| id   | select_type | table | type   | possible_keys                                    | key     | key_len | ref                       | rows | Extra                 |

+------+-------------+-------+--------+--------------------------------------------------+---------+---------+---------------------------+------+-----------------------+

|    1 | SIMPLE      | g     | ref    | einweisungPfl,BezeichnungID,ClusterID,Status,del | Status  | 7       | const,const,const         | 1157 | Using index condition |

|    1 | SIMPLE      | rf    | eq_ref | PRIMARY,Wache                                    | PRIMARY | 2       | func                      |    1 | Using where           |

|    1 | SIMPLE      | f     | eq_ref | PRIMARY,Status                                   | PRIMARY | 2       | func                      |    1 | Using where           |

|    1 | SIMPLE      | w     | eq_ref | PRIMARY,Ressource,Shop,Strassensperrung          | PRIMARY | 2       | func                      |    1 | Using where           |

|    1 | SIMPLE      | b     | eq_ref | PRIMARY,Hersteller                               | PRIMARY | 3       | qmsystems.g.BezeichnungID |    1 |                       |

|    1 | SIMPLE      | h     | eq_ref | PRIMARY                                          | PRIMARY | 3       | qmsystems.b.Hersteller    |    1 |                       |

+------+-------------+-------+--------+--------------------------------------------------+---------+---------+---------------------------+------+-----------------------+