It is the humidity and not the temperature drop. Temperatures temporarily dropping during lockdown won't really affect the hatch unless you get ridiculous about it.
What can happen is that the humidity CAN drop enough that a chick that has pipped dries out and gets stuck to the membrane. It is glued in position and cannot move to complete hatching. I've done it before. I know it CAN happen. I know you should not open the incubator during lockdown, but sometimes you have to deal with problems.
Eggs that have not pipped are not very likely going to have this problem. The ones that have pipped are the ones mostly at risk. And not all eggs that have pipped will get stuck. For me, when I have opened the incubator, most don't get stuck, whether they have pipped or not. It is not something that happens to each and every chick each and every time. It is something that CAN happen, so many of us recommend you avoid doing something that CAN put your hatch at risk.
Not all incubators are the same. Some lose humidity faster than others. Some regain humidity faster than others. Some of us haved drier conditions in the room or something else that causes the humidity in the incubator to drop quicker than others.
I know I am shouting by capitalizing CAN, but many people seem to think that because they violated it and got away with it the recommendation is always rubbish. Like a whole lot of other recommendations on this forum, they are things that CAN happen, not absolutely will each and every time. They are intended to improve your odds, not guarantee success if you follow them to the letter and not guarantee total absolute failure each and every time if you don't follow them exactly. Some people take these things as guarantees. They are not. They simply improve your odds of success.