With a fresh vanilla map and only the OneTileMining mod, it works. I am going to start eliminating other mods, but this appears to be a mod conflict.
Here is the info / Link to this mod: https://forum.wurmonline.com/index.php?/topic/167518-released-coldies-compilations/
One Tile Mining
7th August 2020 major update. v 4.5
# surfaceminechance true to enable changing the chance code.
# default server code is Math.max(0.2f, (float)mining.getKnowledge(0.0) / 200.0f);
# make sure to have a ; at end and dont use any ' or "
# if you want pure % regardless of skill its a float value so do minecode=1.0f; for 100% minecode=0.5f; for 50%
surfaceminechance = true
minecode=1.0f;
#alters speed for mining action to use level INSIDE a mine.
levelfactor=0.5f
#enables mining and concrete usage with just 1 tile of rock exposed
onetilehook = true
# true enables lower action on surface rock
loweraction = true
New 'lower rock to flat' action
Activate pickaxe and right click a rock tile on surface, click lower rock to flat.
This action will start to lower the 3 highest corners down to the lowest corner.
It uses your server timer settings for surface mining, so skill, ql and so on affect the time, it is rounded though to nearest second, so 2.8 would be 3.
All the usual restrictions will apply as if you were just surface mining.
The range on the action is increased to roughly 3 tiles, this allows for easier dealing with steep slopes and also ability to move to another tile when you have 100 rocks where you are standing.
https://1drv.ms/u/s!AjnBltNOSFjCgkfWbCn02pAkzD1S
OneTileMining conflicts with SpellCraft Quarry spell in terms of success rate, however OneTileMining allows you to surface mine rock corners without having to expose all 4 adjacent tiles to rock.