Interesting. I've found the same thing for years, & have always assumed that either I just didn't understand the Key Command significance, or that there was a glitch that stopped Logic interpreting the Space Bar command under certain circumstances.
Logic is so
context-dependant that it is hard to tell when something is or isn't going to happen when you perform a certain action. A little more definitive causality wouldn't come amiss, IMHO…
Note that the Stop command & the command activated by default by hitting the Space Bar are in fact
slightly different (see below).
Anyhow… try this for a long-shot:
_Space bar acts as Sustain Pedal_
Should you wish to use the Space Bar key command (usually Start/Stop), rather than as the Sustain function for the Caps Lock Keyboard (when active), you can disable the +Space bar acts as Sustain Pedal+ option.
You can also look here for unusual occurrences:
_Key Commands Window_
The Key Commands window can be opened by selecting +Preferences > Key Commands.+
_Now for the nitty-gritty:_
The +Stop Goto Left Locatorkey+ command stops playback, and moves the SPL to the left
locator position.
The +Stop Goto Last Play Position+ key command stops playback, and moves the SPL to the last position reached with a positioning command, or direct positioning of the SPL, using the mouse or Bar Ruler. _+The default for this command is the space bar.+_
See? So if that command was inappropriate (e.g. due to Song Position confusion), the Space Bar wouldn't have the expected effect…