Try using the FP.Get Image invoke node instead.
I don't think there's a trick to get the whole tab just using the builtin method. But you could use a workaround, maybe.
Put the tab control in a container of some kind (another tab control, a subpanel, or block it off with a couple of splitter bars). Make the container transparent so it doesn't look weird with the extra border. Then tell the container to take an image of itself, and all it will see is your tab control.
I do not have access to newer versions. It is not clear to me where to report bugs. I have searched and found known bug list for 2017 and it doesn't seem to be listed or for 2018. Doing a search of "how to report bugs" on the NI page search bar yields bogs of unrelated results such as "How to buy..." and every other mention of the words "How to". I would be glad to report it. Can you point me to a link?
It is not clear to me where to report bugs.
If your business depends on it and you have SSP, you can always file a service request. They will evaluate the severity, put it in the bug database with a reference number, and possibly offer a workaround.
However, typically it should be sufficient to post a link to this discussion here in the annual bug thread and it will be seen by NI eventually. In the meantime, forum users can also do some casual testing or search for workarounds.