I recently came across a scenario wherein an eVar variable receiving the value of 0 (numeric zero) was classified as 'unspecified'. I searched for documentation to get this fixed and came across two possible solutions.
1. Implement alternative values (replace '0' with 'zero')
2. Using some processing rule
I came across a possible solution which has been fixed for my test scenario and is working as expected. I concatenated with an empty character whenever '0' was encountered which solved it.
I used a soft hyphen character (https://www.online-toolz.com/tools/character-map-unicode-symbol.php?chid=00AD)
Hope this helps in your scenarios.