A few more details:

1) Many many devices show up in the pairing list. Most of them are "unknown or unsupported device" and quite a few Tile devices. The list changes frequently, maybe every second.

2) Occasionally, but infrequently, the LG device will show up, but only for a second or two, and not long enough to select it. 

3) When I paired the B&O H8, it is possible that it was in an environment where there were not as many other devices present. 

3) I think this could be a user interface problem. A solution might be to suppress all of the "unknown or unsupported devices", and I suspect that the LG might appear there. It seems that legitimate devices are competing with these others in the list. 

The problem with too many devices appearing in the UI is being fixed in  http://crbug.com/817599 .

You are right that maybe your use case of LG HBS-780 headphones coincides with noisy Bluetooth environment. Could you confirm by using LG HBS-780 in the same environment as B&O H8? If the bug is not reproducible that way, that means the fix in  http://crbug.com/817599  is most likely enough to solve your LG HBS-780 problem.

Ok. I am on travel until next Wednesday. I did this in the GWC-3. I will
try to pair both of them when I get back on 5/2, and update the bug
accordingly.

I tried to pair the B&O H9 in the office. It behaved identically to the LG device. I was not able to pair the H9. 

Thanks for trying this out. It's likely that the spammy list is causing the difficulty of pairing. Bug  http://crbug.com/817599  will roll out in M67, let's see if it solves the issue.

I tried this on M68, and both headphones paired perfectly. Lots less
clutter on the pairing menu in a dense bluetooth environment too. Nice job.

Thanks for reporting and helping verify the solution. I will close this bug then.