As you must be aware of, the WebUI.verifyElementNotPresent keyword once had a problem at v8.6.0 but was fixed at v8.6.5.
I developed a project to check it. I reported that the waitForElementNotPresent has been really fixed.
I changed the verification project a bit, as follows:
//boolean kwReturn = WebUI.waitForElementNotPresent(tObj, timeout)
boolean kwReturn = WebUI.verifyElementNotPresent(tObj, timeout)
and checked how the verifyElementNotPresent keyword at v8.6.6 works. I found that the verifyElementNotPresent keyword at v8.6.6 works fine.
So I believe that the verifyElementNotPresent keyword is not guilty for @qa113’s current problem.