Video Conferencing Outbound Video Doesn't Work

I posted this in /r/sysadmin a couple of months ago and most people said "networking" but couldn't get into much detail. I'm hoping some of the experts here can educate me.

When our employees are in our small office they consistently have the problem where the remote side cannot see their video stream or the desktop/application they are sharing. However they can successfully see the video and desktops/apps presented from the remote users outside our office. Audio is usually fine both ways. When our employees are in coffee shops, hotels, home, other offices, and so on they do not have this problem. Employees inside this same office cannot see each other's video either. It happens with employees BYOD devices as well, personal laptops, personal cell phones and tablets, etc. Also, if employees disable wifi on their BYOD devices and use 3G/4G/5G video works again. It's happened with 1 employee in the office trying to video conference as well as 5-6 at the same time. Time of day doesn't make any difference.

The internet connection in our office is pretty good, 50Mbit circuit. Speed tests report good results, comparable to the coffee shops, hotels, etc. Our network link isn't saturated that I can tell. Download speeds are good.

It happens with all video conferencing apps. Zoom, WebEx, Skype, Facetime, Bluejeans, MS Teams, Google Meet. The worst part is that none of those apps report any problem, and none of them see to have any way to dig into the weeds to see what he problem might be.

Our perimeter firewall is basic and only does layer 3/4 filtering. BSD based running pf firewall. No "NextGen" filtering, no application filtering, no layer 7 anything. All outbound is permitted, NAT'ed out our firewall's external interface.

I got a couple of good responses in the /r/sysadmin thread about STUN/TURN, WebRTC, and UDP hole punching. What would I need to change on our firewall to see if it is somehow interfering with these things? I don't even know how to set up a test scenario or what variables to change to start ruling things out.

Has anyone experienced this? Any help is very appreciated!