I do use btrfs since several years (maybe 2012). This indeed is an installation from 2017. Therefor it can’t be default. 
The actual btrfs layout is the result of 5-6 manual rollbacks. (Whilst i had to learn a lot about btrfs)
- The snapshots are taken by snapper
- When updating by snap-pac
- Timeshift does not fit my needs
I not only want to be able to rollback the system, but also want to undelete / compare files with previous versions.
Snapshots in btrfs come with no cost, if you don´t forget to release them. The releasing is also automatic by snapper.
But as you said the timestamps in /boot (27.) are after the rollback (26.) . So i must have updated the kernel after the rollback with mhwd-kernel.
Then this was a partial update 
only including the kernels. And there must have been the problem. 
I don’t like to be not up to date, but i first have to get the update running with my GUI setup (multiseat). 