Thank you for posting the results of your thorough investigation!

Do you estimate that this problem may occur on a fresh installation of 15.5.6 too or would it happen only when updating from 15.5.5 (or even from an earlier version) to 15.5.6 or from 15.5.6 to a future version of WS?

Also, in what OS version(s)/build(s) did you detect the problem?

Finally, do you consider this a WS bug or a WDAG bug?