I can't reproduce this reported race condition in an normal amount of time actually¶

for the records: After which tasks has the bug appeared?¶

1. with Admin #1 create a page (in Chrome 64.0.3282.186 macOS)
2. with Admin #1: create a second admin user (in same browser session)
3. open a incognito browser session (same browser)
4. login with Admin #2 and open page module
5. voilà

¯\_(ツ)_/¯ we all love race conditions

The issue is not reproducible anymore on current master. If the issue pops up again, please feel free to re-open the ticket or ping me.

I've managed to reproduce it.

Data coming from TYPO3 is broken, the "prefix" field contains sth like:

"<span class="typo3-pagetree-status"><span class="t3js-icon icon icon-size-small icon-state-default icon-warning-in-use" data-identifier="warning-in-use">↵    <span class="icon-markup">↵<img src="/typo3/sysext/backend/Resources/Public/Icons/warning-in-use.png" width="16" height="16" />↵    </span>↵    ↵</span></span>" 

Seems the issue occures when there is another user editing the page in the same time.
As the "tip" field contains

"id=5 - The BE-User 'xxxx' began to edit content on this page 6 min ago." 

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56033

Applied in changeset 97deaa23fb24ad6da34adfb5a9ddf6533f3ea166.