Not sure Newcastle keeping him would be irrational. There have been instances before where clubs dug their heels in and re-integrated player. And if no deal this summer, I'm sure Isak will be back across the picket line soon enough.
Not handled well by Newcastle for it to get to this point, and if they were even half competent at signing replacements, we probably wouldn't be here. But can understand them now refusing to let him go in the circumstances.