Locking doesn't lock

I can edit content that should be locked, which shouldn't happen.
Steps:
I install and activate the module
Using two browsers (Chrome, B1, and Safari, B2), both with JavaScript on and logged into two different user accounts, I navigate to the test page
I click Edit in B1 and enter some text in the Body text box. The expected Content is Locked message appears
I click Edit in B2 and enter some text in the Body text box. No message appears
I click Save in B2 and it saves correctly (verified this with Firefox, B3)
I click Save in B1 and it comes back with the unexpected Changes Cannot be Saved message (re-verified no change saved via B3)

This worked similarly with or without Javascript enabled in one or both B1 and B2.

Let me know if you need more info.