When you hold the line and activate this ability, if you then move the strike will still trigger when an opponent moves into range
"For 4 turns, the first time a visible opposing champion moves into this champion's range, this champion makes a free basic attack against that champion. If this champion has no basic attack, it makes a Physical non-basic attack instead. This effect is hidden until triggered." Nothing about whether the champion using it can move or not. So feel free to abuse it!
Oh, you hadn't mentioned that part. This is an ongoing issue with different abilities, I'm not sure when it will be addressed, but I definitely agree with you on it. The reason it happens is because some abilities, like this one, add an Effect to the champion using it, so even if the ability gets removed, the effect is still there. You'd have to Dispel them.
Yeah I can see why it happens, and it seems like it would be very tough to fix (if at all possible). Hell, not even 100% it is a bug.
I think of it working similarly to command dodge granting the dodge condition to champs without the ability. I'm pretty sure it's WAI.