Dell has released A06 BIOS version for the XPS 13 (9333) model. The release notes (available here) are extremely light on detail, however I can tell you it does seem to resolve (at least so far for me, and others) the issue with unreliable suspend and resume!
It doesn’t seem to resolve the bizarre problem with delayed brightness key controls though.
For those of you tux lovers out there, the upgrade is simple, grab a good USB stick (the cheap ones tend to not boot correctly), install FreeDOS via unetbootin, download the BIOS image .exe file, copy it to your USB stick, boot the USB stick, and the execute the .exe, the BIOS installer will start, walk through the prompts and wait for it to reboot your system.
UPDATE:
Well, sadly after a few days of testing, it would seem the bug still persists within the BIOS which results in lock up on shutdown. It took considerably longer to get there, however it’s still present.
Hi, I don’t know if you have received any trackback, but the suspension bug is related to the intel mei and mei_me modules.
[1] http://bartongeorge.net/2015/02/23/update-2-dell-xps-13-laptop-developer-edition-sputnik-gen-4/#comment-89299
[2] https://bugzilla.kernel.org/show_bug.cgi?id=86241
Hi and thanks for the suggestion. I had actually already tried this and found that, in my case, the issue was a DPMS script that was running on my system to keep the screen off.