What, do you expect her to keep you updated all the time on why she's not really able to text you right now? That sounds like a full-time job in and of itself.
I could understand feeling this way if you were having a serious conversation and she just dropped off the face of the planet with no explanation and no response ever, but like Mr. Boat below me said, texting isn't really a formal communication outlet. Yes, it's nice to reply, but in an overwhelming majority of cases, totally acceptable to not reply right away, especially if you're, I don't know, playing soccer or something at the moment.
I don't think it's disrespectful. A little frustrating, yes. But people rarely text bye in text conversations anymore so it's nothing to get worked up about. She at least texted you afterward which shows that she does like talking to you but sometimes needs a little break. Also, I used to avoid saying bye in texts just so I could pick the conversation up later and not have it be goodbye for a long time.
Are you serious?
Okay obviously no one has told you texting is not formal communication, most rules of formality don't apply because it is a form of messaging. Introductions, salutations, closings, none of these are necessary. If you want to have a real conversation that actually develops your social skills do it face to face.