Parenting is rarely a job with a 50/50 balance.
Those who co-parent with a partner or ex often find that there are times when the workload falls more on them, and that can be for any number of reasons. Maybe the other parent is sick, has some kind of social engagement, or needs to do some work. There is a whole list of excuses that some parents feel entitled to use as a get out of jail free card, and sometimes they have more of a right to do this than others.
Naturally, this can leave the parent who has to take on the childcare in their absence feeling like managing their own schedules is going unappreciated. Good parenting is all about teamwork, and if one of the adults involved is not pulling their weight, resentments are inevitably going to arise.
The reality is that being an involved parent, especially in the earlier years of life, requires some level of sacrifice that means other things cannot always be prioritized. It is unfair that the world doesn’t always take into consideration how parenting duties fit alongside other ones, but a responsible mom or dad does their best to work around that.