Remove the FON from Ironhead in a copy of vanilla CS (or maybe make it FOB) and see what happens.
(Aside from the ME tiles appearing.)
I've looked over CS's script for Map 31 (where Ironhead is fought) It doesn't have <FON or <FOB in it's script (unless I've misunderstood something):
#0090
<MNA<CMU0018<UNI0001<MYD0000<FAI0000<END
#0091
<MNA<CMU0018<UNI0001<MYD0000<FAI0001<END
#0092
<MNA<CMU0018<UNI0001<MYD0002<FAI0002<BSL0000<END
#0093
<MNA<CMU0018<UNI0001<MYD0000<FAI0003<END
#0094
<MNA<CMU0018<UNI0001<MYD0000<FAI0004<END
#0095
<CMU0023<FAI0000<END
#0100
<KEY<FAO0001<CSS<TRA0047:0091:0008:0009
#0200
#0210
#1000
<KEY<BOA1000
<ANP0200:0010:0000
<ANP0210:0010:0000
<WAI0300<FAO0002
<UNI0000<CSS<EVE1001
#1001
<ITJ0018:1010<FL+0160<EVE1002
#1002
<CMU0000<FLJ0836:1003<EVE1010
#1003
<FL-0836<FLJ0851:1010<FL+1042
<EVE1010
#1010
<NCJ0336:1011
<TRA0015:0410:0016:0012<END
#1011
<IT+0036
<TRA0015:0410:0016:0012<END