thanks for using our forum! (btw we moved your request from IVION to the M6/VLX section)
Tunnel mappings are always the most difficult situations for the system. Through your description there seemed to be indeed a SLAM lost when you noticed the VLX icon staying static in the screen, and a sensible decision to have a retake after noticing.
However, I'm not really sure of the features effectiveness in the tunnel. Just from the quality map it looks there are features, but in a repeated manner every 2-3 meters. This could be easily also a confusing environment for the SLAM. A specific feature is for example a barrel on the current point and after a few meters a chair (another unique feature) etc.
In the post-processing stage, you can also try to select the outdoor preset for processing, this might also help to correct the original mapping's SLAM break.
I hope the above points can help to some extent. If you would like to deeper investigate on this mapping data, we can also move on to a support case. There we could check deeper with your original data, processed data etc. to see if there's anything to tweak in mapping and processing.