I found a workaround by simply adding another semicolon (my column delimiter) at the end of each line in my csv. Worked like a charm.
Still don't understand why this issue appeared as even trying with a normal string in place of the JSON object in the csv I got the Illegal character inside unquoted field error. My other indices work well and have a similar ingest pipeline and didn't require the additional column delimiter.
Other solutions I tried such as this one didn't work for me.