The problem is: ISO 8859-1 should be used for properties file. It seems that Optifine is able to read both UTF-8 and ISO 8859-1 without issues, but .properties file encoded with ISO 8859-1 will not work with CIT Resewn.
Example of properties that work with Optifine but not with CIT Resewn:
lunettes_icon.txt
(The extension was changed to be able to upload it on github, but it's a .properties in the resource pack)
As we can see here, CIT Resewn expect UTF-8 encoding for properties file: https://github.com/SHsuperCM/CITResewn/blob/ad5ffa41a5e34c9baf897e9178e6d9b4bd6d3ed8/src/main/java/shcm/shsupercm/fabric/citresewn/pack/format/PropertiesGroupAdapter.java#L23
The problem is: ISO 8859-1 should be used for properties file. It seems that Optifine is able to read both UTF-8 and ISO 8859-1 without issues, but .properties file encoded with ISO 8859-1 will not work with CIT Resewn.
Example of properties that work with Optifine but not with CIT Resewn: lunettes_icon.txt
(The extension was changed to be able to upload it on github, but it's a .properties in the resource pack)