Per this closed bug thread:
I can confirm that this bug exists.
Using r99 - free edition.
No plugins or extra behaviors.
Switching to Enable WebGL = On removes the problem.
Switching to Enable WebGL = Off shows the problem.
[ I tried to link to a capx zip on another server - and got the spam detection warning. ]
You can duplicate this problem by taking the
Particle explosions.capx in your Examples folder and
setting Enable WebGL = Off
and the event sheet to
System -> Every 1.0 seconds
-- System -> Create object Particles on layer 0 at (random(400),random(400))