It's kinda hard to explain because it is counter intuitive, here is the reproducing sequence, paste this in aterm:
PS1="12345678901234567890123456789012345678901234567890123456789012345678901234567890"
ls -al
echo This line will be erased when you press Ctrl+U
Now, type some garbage and ctrl+U, the last line gets erased.
Turns out this only happens in aterm, if I load bash.tcz and use bash this doesn't happens.