To reproduce:

1. Create a map of somewhere, put it in an item frame (so you can see it)
2. Set a decoration on the map, such as

/data modify entity @e[type=item_frame,sort=nearest,limit=1] Item.tag.Decorations set value [{Id:"_",type:1,x:-128,z:0,rot:180}]

1. Note that type 1 is the "frame" icon — it is visible on the map.
2. Change the decoration, such as

/data modify @e[type=item_frame,sort=nearest,limit=1] Item.tag.Decorations set value [{Id:"_",type:5,x:-128,z:0,rot:180}]

1. Note that the map still shows the "frame" icon, not the "Target Point" icon.
2. look at the map data:

/data get entity @e[type=item_frame,sort=nearest,limit=1] Item

. Note that the Decoration NBT has type 5, not type 1, but type 1 is what's displayed.

1. For extra credit, use "<Opt>pick" to put an exact copy in the inventory, and put it into another frame. The map will still show type 1, but have type 5.