I've been in touch with KSP support and they know about the issue and are trying to recreate the problem but are having issues. I sent them this video to illustrate the issue, but you might consider sending them an email to see if there's any other data they need.
[email protected]
Ok, that works, however it's super annoying. Add a few parts then restart the game? Also the restart takes quite some time, so is super annoying. Hopefully a fix comes soon.
Same issue here on my PS4 Pro. I got this game after hearing all the great reviews and having my daughter come back from space camp loving the experience. I thought what better than to build your own space program. However this inability to add/edit items on the rocket is frustrating to the point of making the game unplayable.