I have experienced the same thing... and B or spacebar might save a few seconds over scrolling the touchscreen - but that means I have to slide open the keyboard, giving back the couple seconds saved!
Anyway - it's a bug for sure. There's no reason opening a new text would put you in the middle of a conversation that might be hours / days old. Hopefully it will get fixed in a later release.
Anyone found a setting anywhere to always open at the bottom of a text conversation?