When the upper is opened, the back of the carrier comes off the face of the buffer, and the buffer lightly crashes forward into the buffer retaining pin. If the buffer retaining pin channel was milled too far forward to begin with, the very face edge of the buffer would be beat to hell, since it would be the buffer retainer holding the buffer in place, instead of the back of the carrier when the upper is closed.
Now since we do know that buffer when the upper is opened will lightly crash into the pin, so in a give time, the buffer retainer pin will oblong the channel slightly.
But, we are back to square one, being if the receiver extension is worked so the face of the tube is kissing the center pin of the retainer (and it forward in it's channel) with it set correctly, then there is no reason to replace the lower receiver.
To bottom line it here, you have lower receivers that have not been thread start indexed all the same, then even receiver extensions that are also wildly start point threaded as the start point of the thread to the tube. Short of if both the receiver and the extension are produced from the same manufacturer to hold this control, expecting that any receiver extension on a telescoping stock is just play and play to any lower receiver, is beyond wishful thinking. Here, in regards to such matters, about the only time you get plug and play receiver extension, is with a fixed stock receiver extension, since there is no butt stock orientation on the tube, and it just a mater of the face of the end of the tube, and the stop collar distance from it; with threading start point on the tube not mattering.
So again, mod/shorten the end of the telescoping buffer tube so you can spin it in to do these four things.
1. it indexes the butt stock for the correct orientation.
2. the end of the tube kisses the center post of the buffer retainer with the buffer retainer all the way forward in it's channel.
3. Since the bottom face of the tube has been correctly timed for the two above items, make sure/double check to see if the top of the tube needs to be worked so it flushed with the inside face of the lower receiver barrel thread socket so not only can the upper/back of the carrier clear the end of the tube, but so the buffer front side bearing edges are always contained with in the tube.
4, And before you start doing the math of just how much the face of the tube needs to be faced, make sure that when the B/C and buffer are moved all the way rearward, the face of the bolts max travel backwards ends up with the face of the bolt from 1/4" to 3/8" in front of the back of the ejection port.
So to take it back to the topic I listed, here should be your end results when you have installed the receiver extension correctly.
Note, Op used a file, while I use a mill to end up with a smoother edge at the facing, but these is just a final appearance item, than a needed correctly fitting function item as he has acheived.