@jzchen I've had much the same experience as you've described more clearly here. For context, all of my devices are hooked up via ethernet to one of the 2 BE63 router APs/Meshs.

My situation has been that when I initially hooked up the BE63 to my BG gateway, everything worked pretty much flawlessly. My Xbox (Series X) was showing great speeds much more similar to what I'm supposed to be getting as opposed to the BG gateway and yet I still was experiencing some strange behavior. The Xbox was showing a strict NAT and my Plex server was still not accessible outside my Local Network and showed "Double NAT detected". This was all while UPnP was enabled. After some attempts at Port Forwarding (didn't change anything) checks on Public IP and Open Ports, I decided to do some cable rerouting and reconfiguring as well as some router and gateway restarts. 

I then experienced some new issues and some resolutions to my old issues. Suddenly the Xbox was showing a fully open NAT, while my Plex server was still showing that it was unable to be accessed remotely, although my Steam Deck was able to stream from my Xbox remotely, meaning that UPnP was working somewhat, but the BG firewall was still getting in the way. This behavior has been inconsistent and highly volatile until now. Regardless of my testing or different scenarios that I attempted, I have been unable to replicate any specific outcomes with reliability. It does seem to be mostly related to restarts of the gateway or the routers and the timings between either of those. 

Another additional problem that I've been having is that IPv6 sometimes does not show as enabled despite me enabling it within the BE63. Again, online checks through IP checkers, Port checkers or IPv4 or IPv6 checks show nothing has been solved (ie, no open ports, no IPv6, etc). My Xbox mostly shows an Open NAT with IPv4 and IPv6 but will occasionally revert to Double or Strict NAT and IPv4 only.

I tried speaking to AT&T customer service about this and they told me point blank that I know more about this than they do (overseas support, I have been unable to reach somebody American) and that they do not have the advanced training in such technical areas to be able to help me. They did also claim that my IP pass-through was working perfectly and that I shouldn't have any issues (after accessing my Internet connection) but obviously this isn't the case. They instructed me to seek out Geek Squad assistance (lol I balked at that but I don't really have other options atm) so I did through their in store desk who told me to call up their number because I wouldn't be able to get something done in store. They would have to send the technician out to my home for an initial consultation fee of (won hundred fivty...this forum doesn't allow monetary posts apparently, lol) which is when I declined to do that.

In conclusion, I'm having the exact same issues, but because my speeds are much better and I needed the additional Ethernet ports and customization options I have decided to keep the BE63. It really seems as if it's up to AT&T to fix either their poor software protocols (I can't believe the gateway settings are still basically the same as they were when my parents had U-Verse Internet over a decade ago) or their lackluster hardware (the gateways are garbage).

I'll post if something changes in the future but currently sol on this.