In my save file I now have a fast inserter that has the length of a long inserter...

It seems to be a problem of copying one behavior onto the other's. As in I am capable of doing this with ease

Simply place both side by side, a fast and a long and use shift left on one and then shift right on the other. It will result in the length of one copying over to the other. Yes, this works on both. You can make any inserter long OR you can make the long inserter short.

It is a comical bug but a bug nonetheless.

It's not technically a bug, that's just how inserters work in Factorio. They can either be adjustable or not adjustable and since all the throwers have to be adjustable for the range changing stuff, their pickup range is also adjustable. I guess I could detect the pasting of inserter settings and revert the change if it's further than it's "supposed" to be but I don't think it's really hurting anything.
Also if you have Bob's adjustable inserters or a similar mod you could manually set the throwers like what you described so there's always work arounds even if I try to block it lol

I know about bobs inserters but I find them quite cheaty in more vanilla like mods. But don't worry it's not like you have to fix it. It is quite a funny bug, having a long fast thrower inserter. I posted it on reddit and people were quite confused. If it's too much work it could easily be left in the game or even called a feature. Just wanted to mention it because it is a bug in it's origin, being able to make a long range inserter small and the opposite for all