You have a filter with too much restriction for this kind of fan, is why it was sucking air in the front. The fan is trying to move air, but can't move air through the filter from the rear very well, so it moves air around from the front instead. If you could get the shroud around the blade, instead of over front of the box, that would (be difficult but it would) help. Getting more surface area on the filter, or just increasing the number of filters would also help. The shroud you have is a band-aid... which, if it works, I entirely support! If it works, it works.
Living in the allergy capital of the universe, I have rigged up a DIY HEPA FFU for use in the house. My work was going to pitch new-ish 20" square 3" thick HEPA filter modules, and I slapped a 20" box fan on one. Not much air flow, too much static pressure. It did what you noticed, sucking air from the wrong side. I made a box with 3 of the 20" HEPA filters for sides, cardboard for bottom and front, and the fan on top. Tape to hold it all together. Over top of this, a big cardboard shroud directs air toward the front panel, because it was circulating air from the top to the filters on the sides. Now it blows in one direction, LOTS of cleeeeeean air. Recommended, if you can spare the space. And can stand to look at it.