Kodi18, based on what?

I wonder how a switch to a new version works? Or should I say, is it based on the current state of Kodi17 or is it already in a new branch long before that?
The reason I’m asking is that things that were working in Kodi17 isn’t in Kodi18. So maybe Kodi18 is missing some things until some point where all Kodi17 features is merged?

Besides the EPG not closing on channel change, also when searching for subtitles the last search is not saved, as it was in Kodi17. So I just wonder how the process is when moving to a new version.