I could fix it, and it will be released with version 12.0.16 (bug XIVY-19036 Improve the parsing of dates from past log files in Engine Cockpit).
However, this is only a UI bug. At the moment, all the logs are under the current date instead of the correct one. It's not ideal, but at least you have all the logs. They are stored correctly on the file system.
To be precise, we only parsed the date correctly from logs ending in .log.gz. After the fix, we respect all logs ending in .gz.