I can't find that in preferences? There is the normal toggle in the options menu, "on stop rewind to now marker".
Everything usually works as normal, spacebar will stop and return to the marker, or in case the other option is checked, the marker will move along playing and spacebar will stop on the current beat. It is only jumping to zero when spacebar is held for too long, or in my case accidentally double clicking spacebar when starting the song. (something that can be hard to avoid especially on big arrangements)

I had not thought about this, before OP mentioned it. Never happened until I tried to replicate.

tlw
Holding the space bar will send Sonar multiple space bar clicks. How long you have to hold it for this to happen will be controlled by the Windows setting for character repeats when a (PC) key is held pressed.

Very good point. That is how repeating keys would work.

This behaviour drives me crazy!
It has often happened to me since a long time. Especially when I wanted to listen to a song part several times and hit the space bar for the stop and the restart to quickly in succession (like a double click). Hence each time the jump to the project start stops my musical inspiration totally! That's why I was searching for a solution and came across this thread. But it seems the problem was recognized a long time ago and then it has been ignored until today!
 
Marc