I think we assume the adaptability and resilience of children too much. Let's face it, at the time, they don't have much choice, do they? It's often later that we see the consequences of what happened in childhood, when they have the least control over their surroundings.
Parents can 'model' all they like, but even in lockdown-type situations, parents get to call the shots, from a child's point of view. Their situation would not be viewed by a child as the same, particularly a child not old enough to understand the restrictions.
Look at it from the child's point of view for a moment. However short a few months (even if it was a few months, others have pointed out the reality in terms of disruption was longer) may seem to you, to a four year old, it's a very long time. If your four year old has been used to being in a busy nursery, or seeing Grandma every day, or going to playgroup, whatever, suddenly that's changed. They don't fully understand why, they haven't enough concept of time to view this as a short interruption.
In addition, Mum and Dad may well be behaving differently, however hard they try not to. In the short lives of children, consistency matters. Change is, actually, a much bigger deal when you are young because your experience of it is limited and you can't control any of it.
It's not the child being at home that is the problem, necessarily. It's the change. From a small child's point of view, they've spent their whole life having this unchanged routine, seeing these people, going to that place, etc. Now, their world has shrunk considerably. Adults understand this isn't permanent, etc. Small children don't, not in the same way.