I believe this is a bug Ivanti are aware of; we also noticed this when we started using Incidents and raised a support ticket with them, 31st August. This is currently 'Referred - Internal' and we received the following response:
"I have tested the issue you have described I got the same result. I have escalated this to our technical lead to investigate further. I will return to you as soon as I get an update on the matter."
Unfortunately in the meantime it would be a case of advising staff to minimise once finished. Thinking about it now, you could maybe make the Incident workspace tab unclosable but that would likely result in many Incident tabs so wouldn't be ideal.
Sorry I couldn't be of more help!
Our partner provided update:
"I have clarified with Ivanti again and was told that the reported issue was not able to be addressed in 2017.3, so it has been pushed back to 2018.1, the next major release.
2018.1 relesae is currently scheduled for April next year."