I can give it a swing. there's something out there called macam, which is supposed to support all manner of webcams for this. It does mention that this 5k monitor does have a camera, microphone and speakers. You might have to plug a usb-C cable into the monitor to use the built in camera. Doesn't say anything about the Thunderbolt cable... Should come up as a choice in FaceTime, under the Video setting. If you choose "Use system setting" it should pick the one built into your MacBook Pro, so, not that. You could try doing a reset smc/pmu and zap pram on it, shut the LG 5k monitor down, and leave it shut down until you have connected the USB-C cable, done the reset smc and zap pram stuff, and then turned it on again. Although I don't have your model and resolution of monitor, I do have an LG monitor, granted, it's only 4k, and there's no Thunderbolt 3 on it at all, which is fine by me as my 2011 Mac Doesn't have Thunderbolt 3. However, for data transfer, charging and the like, including monitor updates, a USB-C cable must be connected between your LG monitor and your MacBook pro 2019, along with the Thunderbolt 3. At least that's how it is for me. anyways, give it a go, and let us know how you make out
cheers
John B