The exact same issue as this: MrCrayfish/Configured#55
In that thread you said you fixed it, but I'm still having this issue more than a year later. Using the latest 1.18.2 version.

If you meant you fixed it from 1.19 and onwards, would it be possible to release a fix for 1.18.2 as well? I've tried adding this line:
[modproperties.alexmobs]
catalogueImageIcon="alexmobs_icon.png"

to both the Citadel and alexmobs config as well as placing a 1x1 .png file in the main directory of the .jar but neither methods fix it.
Using the latest 1.18.2 version.

Fixed it.