Thank you for your post!
I was able to reproduce your issue with FileMaker Pro 15.0.2 (Advanced) on Mac OS X 10.12.2 and Windows 7 Professional. To reproduce I followed these steps:
1. Created "test.fmp12"
2. Created a field and set the font Style to Underline
3. Typed in a field and included Tabs
I did receive non-underline space where the Tab exists. It seems this behavior is common to both Operating System with FileMaker 15.0.2 and older. I will forward this for review, but you may also want to post this to Product Ideas for a Feature Change. I will post back here when further information is available.