Similar experience: I had an 8 Plus and an X, both Intel. I switched to an X with Qualcomm later and can share the exact same experience that you had by going from the Intel modem to the Qualcomm one. However, this time around it looks like it's the antenna design to blame. It has been posted many times based on the Apple technical documents shared and some other findings. In this case you can't really fix/improve it with software. It's just not a good design. Only Apple knows why they've decided to change the antenna design of the X.
I haven’t seen this mentioned anywhere yet - only the modem differences as an issue. Any links?
If it’s antenna then maybe we just holding it wrong again (anntenagate 2.0)? I guess if that’s the case then someone will figure that out soon enough...