Looking into this.

The reason the change wasn't exported was a patch failure:

error: css/css-flexbox-1/percentage-heights-001.html: already exists in working directory

This was probably due to the CL being able to land in Chromium when external/wpt wasn't completely up to sync. I can make a manual PR upstream based on the patch sometime this week (or if you're willing, that's great too). Thank you for bringing this up and I'm sorry this happened.

Ah I see. I can make a manual pull request right now.

(But that failure mode isn't great... nobody would've noticed if I hadn't watched this manually)

https://github.com/w3c/web-platform-tests/pull/6345 -- now I just need a reviewer! (or do I? can I bypass that?)

Ok thanks! I just approved the PR. I'll make a follow-up bug to see if we can make the patch failure mode more obvious.

Bug:  crbug.com/738502 

I believe this bug is no longer relevant. The immediate problem was fixed. Furthermore, both the importer and exporter have been improved since then and there is no known "data loss" issue (like reverting unexported changes in this case) at the moment.

Or in fact, I'd prefer reopening the more generic  issue 738502  which was merged into this CL to discuss if we should turn the exporter red (or send some other kind of signals) when an in-flight CL cannot apply cleanly to upstream.