Sitting for a few days allows the converter to drain. The fluid doesn't pump in park so you need to start in neutral which does circulate the fluid. It's basically normal for that to happen. trans go makes a no yo yo kit to cure that.
Used to be called morning sickness
Will usually get a bit worse over a long period of time like 50K miles. I was told many years back that the condition was due to excess clearance between the converter and bushings. Maybe JK or Tman can confirm or deny that.
Doesn't seem to bother anything as along as the neutral thing is done before dropping into gear as I drove my 66 wagon with it go over 50k miles. This was before the Trans go No Yo Yo Kit was out there or before I became aware of it. I believe they have incorporated the "fix" into all of their shift kits
Feel free to correct me if I am in error
