The patch notes mentioned that library and university research would not stack anymore. It caught the vault, with the library oil and moctezuma no longer working.
I think this might be the source of the change to hold the gates. There is library research (Partisans) that decreases stable first spawn time by 50%. Now that would not stack with hold the gates. So instead of 50% plus 25% for a 75% reduction in spawn, now it would be 50%, and then 25% of the remaining 50% - so only 62.5% total.
That could cut the effect of hold the gates significantly, and that may have been on purpose.
The acropolis is also boosted by library research, so the effect of that could play a role as well.
The question then becomes whether hold the gates is still providing some effect, but about half of what it was before, or if it is actually providing no effect.
It would not shock me if increasing that spawn time was the primary purpose of that fix. The flood of tanks on Hold the Gates bases was extremely unbalanced on defenses. Maybe now it is just a little less overpowered.