I have regenerated all the SDK6/SDK7 firmware images with the new compression option disabled. They should appear on Onedrive/Sourceforce in a little while. The ZIP filename will have beta1b . The firmware version will still show as beta1 - I haven't recompiled the code, simply rebuilt the .trx files with the mksquashfs change.
I recommend everyone to upgrade, just in case there might also be filesystem corruption issues with other models.
I haven't rebuilt the HND models (AC86U and AX88U) because they use ubifs, as well as a newer kernel, so in theory these should be fine.
Interesting thing is the corrupted images will read just fine on my Linux VM, but mounting them on a loopback on the routers themselves will generate read errors while accessing certain files. I suspect it could be a bug in the older kernel, or an incompatibility between mksquashfs and these kernel releases.
I'm not 100% sure this was the cause, but it's what made the most sense (unfortunately, I cannot reproduce the problem every time I build an image). Only testing in the long run will confirm this was the issue.