Hi @Admiraludon,
I have an update. I think that your log files are fine, and they should be actually collected. Even the line with the different pattern should be captured because we have a fallback pattern to capture any line.
Please check if this query returns any document:
GET /filebeat-*/_search?q=event.dataset:mysql.error
I think that the problem may be in the date parsing. Your logs are probably there, but not at the time that you expect. If the previous query returns any document, please check what is the timestamp, and then select this time in kibana discover. You should be able to see your documents there.
I have prepared some fixes to improve timestamp handling in this pull request: https://github.com/elastic/beats/pull/14130