My humble and still developing observations:
For us 35-45 minute nap = undertired. Need to extend A times. DD is also a crazy loud sleepstage shifter, so I can usually see the jolts coming, but nothing I do to intervene helps much, so I usually just freeze and hold my breath until she's gotten through it. It's actually best if I'm not there because with WTS and HTTJ, she would see me and smile and want to play and then it was all over. If she does wake all the way up, I'm just resigned to it, pick her up and go play, usually for another full two hours.
16-20 minute nap, however = OVERtired. If I can't resettle with shh pat, I just snuggle quietly for a while, and she usually is ready to sleep again within the hour.
Clearly not a perfect system, but it's what's working for us right now. Sounds like they eventually just grow out of this.