This takes place on a 7x7 platform, to which I have teleported to the exact center.

It would be expected that a map containing this platform would place the icon at the exact center as well, however this is not true. Looking closely, it can be seen that the player is instead offset to the top-right of it in this case.

While I am yet to conlude this, I suspect the cause of this to be due to how mapicons are handled - while each map icon is 8x8, this marker in particular is implied to have an odd center, but still must be in an 8x8 grid spot, resulting in the icon being off-center as a result. This could be fixed by doing what Bedrock Edition does and doubling the dimensions of the icon, allowing it to be correctly centered. (I'd also advise outright splitting map_icons.png into individual sprites in the process.)