Not quite sure of your question here? Are you thinking of buying one and want to know if it will do this, or do you have one that is doing this and wonder why? l’ve not used one of these. Seen one in use though, and do have extensive experience in using bio and chemical containment systems, so understand how all this works. I’ll therefore try to answer you.
For the former, Andy’s suggestion is a cheap sensible option if this prospect worries you. I have a much cheaper spray booth that simply uses a push in particulate filter and this doesn’t happen to me. I’ve never been able to vent mine directly out of the window, so just allow it to vent into the room with the window open. I do wear a good mask though.
For the latter, breakthrough could be due to poorly fitted filters, though this looks a high quality unit so that shouldn’t be the case, or filter overload.
Overload can happen if the filter is at the end of its natural life or if the particulate challenge is too high (ie too much going through at once).
A HEPA filter like this isn’t a tea strainer mesh type filter. The best way to visualise it is by thinking of the filter matrix as a very deep maze that the particles have to negotiate to get out. The constant twists and turn mean the particles lose energy and momentum and eventually get stuck in corners or blind alleys in the maze, so can’t get through.
These HEPA type filters do eventually allow break through though. As the pockets and blind alleys load up with old particles, the new particles can get to the end of the maze. However, if this happens you would notice a marked drop in the filter face airflow velocity by that point.
I would say the primary design ethos of this booth is to reduce or eliminate overspray, so personally would wear appropriate PPE while using it for anything except water based acrylics. If you really want to eliminate all particles and solvents you would need to look at professional kit, designed for continuous use, which comes with a far larger price tag.
This one looks good to me for hobby use though.