See screenshots:
https://imgur.com/a/pRzcoZx

Now I noticed this code:

int xGrid = Mathf.Clamp(Mathf.FloorToInt((pos.x + halfWorldSize) / gridCellSize),0, maxGridSize);
int zGrid = Mathf.Clamp(maxGridSize - Mathf.FloorToInt((pos.z + halfWorldSize) / gridCellSize),0, maxGridSize);
string extraA = string.Empty;
if (xGrid > 26)
{
    extraA = $"{(char) ('A' + (xGrid / 26 - 1))}";
}

I can be mistaken, but I think this should be 25 (0-indexed) and/or >= 26 in the xGrid check?