The end of the spring at the guide rod cap is twisted. That makes uneven contact with the seating collar inside of the front of the slide. Attempting to install the guide rod assembly will cause the spring to bind and the rear guide rod cap cannot be seated into the slot on the rear of barrel lug.
The other condition is the end of the spring can slip from under the guide rod cap. That will also prevent the end of the guide rod to be seated into the collar of the slide.
There have been reports that the end of the spring has slipped over the guide rod cap and jammed the slide during operation.
It took 16 days to receive a new guide rod assembly from S&W Customer Service in August of 2024. It was exactly the same part.
I did my own "gunsmithing" to fix one guide rod by removing the main spring, cutting 3/8" off the end of the twisted spring, and cut 3/8" off the rear of the spring. That fixed my guide rod spring assembly.
This seems like a failed design and I have not read about a corrected replacement from S&W.
My Sig P365 has a washer under the guide rod cap.
Galloway Precision offers a replacement guide rod. I tried one but it would not allow the slide to go into full battery, so I returned it. Others have reported that they work great.
See attached photos.