I looked into this issue, here is a potential solution for folks who are running into it:

Background)

There are two important values to the mod which are hard-coded into a config.xml file. It is common for people to edit them in order to enable ultra-wide resolutions, but my mod assumes the values are set to their defaults. According to some folks in the Noita modding community, it isn't feasible for me to dynamically update the mod in-game based on config.xml, but I can have you all manually edit the hard-coded values.

Step 1) Find your value for internal_size_h and internal_size_w

There are two places you may have made an update to these two values.
It will be either
1 - in your saved game files: %AppData%\..\LocalLow\Nolla_Games_Noita\save_shared\config.xml
2 - less likely, in the game's files: Noita\config.xml

In either case, navigate to that file in File Explorer and open it with a text editor like Notepad.
You're looking for these two values (numbers will be different if you've edited them):

internal_size_w="1280"
internal_size_h="720"

Record their values for step 2.

Step 2) Update the mod's values

You should probably do this step while the game isn't running. You'll also need the latest version of the mod which I just put out.

Figure out where the mod is installed. Probably, this is in a folder by this path: H:\Steam\steamapps\workshop\content\881100\3251060516\

Adjust the drive letter (H:) based on your Noita installation location. the 881100\ folder is where Noita Steam mods are kept, and 3251060516\ corresponds to this mod.

Navigate to the files\ folder and open fowm_map_render.lua with a text editor (Notepad will do). Near the top of this file, you will find the following lines:

local internal_size_h = 720
local internal_size_w = 1280

Adjust the values of these numbers to match the values that you found in config.xml

Thanks very much!