What records are in your current found set?
When you enter preview mode, filemaker previews all the records in your found set and you can use the "book control" to flip from page to page of your preview. Thus, you may be seeing a different invoice in the preview instead of your current invoice record.
Try isolating your invoice as single record in the found set. You can do this quickly by selecing Show All Records, Omit Record, Show Omitted Only. Now enter preview mode and see if that now preview's correctly.
As of now, there is only one total and I can not flip through any pages. However, I created a new invoice and clicked over to the Image Layout with the portals, looked at preview mode and now only the Line Items portals are showing up and nothing from Invoice Billing Customer portals! The opposite of yesterday!! :(
Also, I created a second invoice and I do now see what you mean how in preview mode you can flip through if there are more in your found set. Thanks for mentioning that!
With the exception I mentioned earlier and filemaker's sliding feature (which isn't a factor here), browse mode and preview mode should show nearly the same thing and data visible in one mode should be visible in the other as long as you don't have the Hide when printing option selected--which you've already checked. The only other way to hide something, is to use conditional formatting and the get ( windowmode ) function to set colors to match the background--that seems even more unlikely here.
Not sure what else I can suggest at this point other than you might try creating a completely new layout to see if you get the same results. If you don't maybe your layout was corrupted in some fashion.