I made a tunnel a few weeks ago and had everything reinforced as well as the floor reinforced and paved and catseyed. I used lava to melt 4 or 5 veins along the way to speed up the process, the lava above ground has been cooled as well and returned to rock. The first location I used lava on collapsed sometime yesterday and this section of the tunnel has been finished for a good 2 weeks now, so this seems to be some sort of bug involving the use of lava.
I would like to know if a Dev would want to look at this spot, I have left the fallen rock in place but would like to clear it out, as it breaks my highway so please let me know if you need to see the offending location.