Filtering the fat on a regular basis screens out particles which cause your shortening to break down more rapidly, but in time the ph will drop anyway. When you do replace fat, do not replace all of it. The leavening system in your mix was developed to operate in normal conditions. Normal, in this case, means that the fat has been used before, and that the ph is lower than that of freshly processed shortening. It doesn't take much time and its a lot cheaper than losing a batch or disappointing a customer.
Mixers do vary , certainly from one manufacturer to another.
But even the same machine, over the course of time, use, and mechanical wear, will tend to vary in terms of its speed. The next time you mix, take a stopwatch and count planetary RPM during your second stage of mixing, then write it down. The problem is not uncommon. The cutter disc, then, retracts into the sleeve, causing the donut to drop, with a hole in the center. Beyond that distance, cut size can be erratic as there will be too much variation between the top disc pushing the batter down, and the lower disc letting it escape.