Finally got around to do the instance again, so modified the MapStyles code a tiny bit. Hopefully it should work for other people too, and not just me.
Also renamed the .dds files to match the pattern of all other map files.
Code:
<ImageStyle Name="exp05_dun_miragul_upper_0" displayname="Scion of Ice (Top)" menugroup="Everfrost (Island)" zonerect="-18, -119, 252, 195" heightmin="-5" availablepriority="1">
<ImageFrame Source="images/maps/map_exp05_dun_miragul_upper_0.dds" SourceRect="0,0,436,506" />
</ImageStyle>
<ImageStyle Name="exp05_dun_miragul_upper_1" displayname="Scion of Ice (Lower)" menugroup="Everfrost (Island)" zonerect="-18, -119, 252, 195" heightmax="10" availablepriority="2">
<ImageFrame Source="images/maps/map_exp05_dun_miragul_upper_1.dds" SourceRect="0,0,436,506" />
</ImageStyle>