I would test/check everything again without the S-View cover
Good point and it's the same with and without the S-View cover which now rules out the S-View as the issue. So, as it's not a S-View problem or one of the Buds Live, it comes down to the phone and/or Google Assistant.
Does it work without any screen lock set?
I just tested with my JLab Epic Air Sport buds (still waiting for my $150 credit so I can get the Galaxy Buds Live - ugh)
Anyway, this is unlocked N20U on Verizon with S View cover; I was able to place calls with phone locked, unlocked, cover closed, & cover open.
Sorry I don't know the solution, but I thought my testing may help you isolate the issue on your device.
I also have the unlocked N20U on Verizon and appreciate the feedback. I hate to reset all the settings but may be headed in that direction.