When I was dropping my 5yo in school I noticed a classmate of hers very hesitantly making his way to his classroom. I just thought it must be the Monday blues. His dad started consoling the child "Son, no one is home today, mamma has gone to office, even I will be going to office today. So, don't feel bad. Enjoy school".
I understood the context because my daughter was affected by the same bug a few weeks back. I would like to call it 'everyone-home-except-me' bug. The cause of this bug is the recent (wait! 2 years is recent?!) WFH policies of a lot of organizations where parents are employed. What happens is that the mother, dad and younger siblings(if any), are home when the kid goes to school and they are home when the kid comes back from school and the child feels left out. It's like all your friends are having party 5 days a week and you have to be somewhere else during that 'happy hour'.
Kids love to spend time with their parents and siblings, at least when they are young. They might act as if they are ignoring you in the park, but they really like it. One useful tip to overcome this 'everyone-home-except-me' bug is to spend a lot of time with that child before and after school time. It sort of works for us.