Text messages getting erased by person your texting!

I noticed this problem in iOS 7 but can't seem to find anything in the forums about this. The problem has continued with iOS 8 too.

If I am already using my phone (surfing the web, looking at photos, whatever) and someone sends a text message I can either hit reply in the small window that pops up on the screen and send a text back or hit the close button and I will text them later. If I choose to reply now and begin typing out my text and that person happens to text me again before I can send my reply text, it will completely delete everything I just typed to show what that person said! It is so annoying and the only workaround is to finish doing whatever I was doing on the phone then go into Messages and continue the conversation there uninterrupted. This same thing will happen if I am texting with Person A then say I get a text from Person B. Person B's text will delete anything I already had typed to send to Person A but hadn't sent it yet if that makes sense! It is so annoying to have to retype something because it was getting deleted before you could send it off!

I really would like to know why its doing this!