Hi @joaocorreia,
We’ve stumbled upon similar and very confusing bug recently: https://github.com/snowplow/enrich/issues/374. The problem was deeply inside libraries we’re using - basically JSONPath lib extracts integers… not as integers, despite in the output still looks so.
On the bugtracker a similar case described (where integers turn into numbers or decimals, not strings) and it got fixed in recently released Enrich 1.4.0. I’m pretty sure that your case would also work with 1.4.0, but need to double-check that.