Sorry, but I have same problem. I posted help message to this board 2 days ago, but MyHeritage apparently does not respond to postings. I noticed under my Administration Tab that Immersive Front Page does not have an icon like the other tools. Looks like the problem is on their end.
There are some limitations to the Immersive Family Tree that we are working to fix as we speak. There will also be updates in the near future which will give you many more capabilities in the Immersive Tree.
For the time being you can remedy this situation in Family Tree Builder. Go to the marriage you would like to see on the Immersive Tree and click on the EDIT button of the person who you want to change the spouse for.
Go to Manage Spouses and change the order of spouses in this window. The first spouse (listed as number one) is the primary spouse or main spouse. This is the one that will always be shown first.
After completing this step, please re-publish your tree. This should take care of the problem.