For the first part of your question - This is due to gimbal lock. You will need to account for Gimbal lock when doing any rotation math. You can find information on Gimbal lock here:
And Quarternion math here:
For the second portion of your question -
What steps can I take to reproduce this on my end?
Does this occur in 4.12 or only in 4.11?
Does this occur in a clean, blank project with no additional content or is it limited to one project?