Based on my own testing, the problems seems to happen if multiple binaries crash simultaneously (like at logout). One crash file gets fully written and the other is incomplete.
If I remove apport from the equation and just get the kernel to dump core files then the core files are always written reliably (should be several hundred MB in the case of gnome-shell).
Repeatedly unusable truncated crash files:
Bug 2012974, bug 2015842, bug 2015140, bug 2012075
Based on my own testing, the problems seems to happen if multiple binaries crash simultaneously (like at logout). One crash file gets fully written and the other is incomplete.
If I remove apport from the equation and just get the kernel to dump core files then the core files are always written reliably (should be several hundred MB in the case of gnome-shell).