An update that may help people in some situations.
I found that my Sabotage (Black Well) maps issue was related to VRAM usage. The Nvidia app had optimized BF6 for balanced performance/quality and set 'Texture Quality' to 'overkill' - the highest setting. My 3070 has 8GB of VRAM and the overall combination put me slightly over the 8GB VRAM ram seen in the graphics settings panel.
When I reduced this to 'High' I no longer failed to load these maps when I tried to join solo. I suspect that many times when people are 'left behind' they may just be hitting a similar problem like this due to bad luck of the map picked.
This did not fix all my problems with being left behind but it did improve it.
The first time match making failure no longer happens.
I still see a random failures when we try to let the game auto-pick the next match. This looks a bit different because your usually stuck at the score screen and the music and sounds from the new map maybe playing but you can't go anywhere. I can 'sometimes' click on 'Join Game' from the party menu when I realize this has happened and it may let me in the game with everyone else. In one instance I got a strange cryptic yellow error code at the bottom of the screen when I tried to do this but it still pulled me into the game.
BF6 really needs a log file that could detail what failed in these match loading cases. I tried to find such a file but nothing stood out.