I suppose this could be a feature to help prevent accidental modification of previously synced and sized subs, but I personally, would prefer to be able to make changes without first having to first select the subs.
I have experienced this on many systems (and have almost pulled my hair out trying to figure out why I can't adjust the sync until I figured it out) including Linux Mint Cinnamon 17.3 and 19.0, Linux Mint Mate 19, and whatever version of Xubuntu I took for a spin.
Currently using SMPlayer 18.10.0 rev 9144 but I have experienced this for quite awhile.
I'm a huge fan of SMPlayer primarily for how well subtitles are integrated into it. This is becoming much more common nowadays, but SMPlayer was way ahead of the curve with this - especially opensubtitles integration. I'm still hoping for subdivx and addic7ed plugins
