CS6 was current in 2012, THIRTEEN YEARS AGO. You have, unfortunately fallen way behind. CS3 was version 6 of the software. There have been 15 versions released since then.
Is that the only reason here? I don't know but I do know that it can't eliminated as the cause. One way to find out would be to take a seven day trial of InDesign 2025 and try it.One way to find out would be to take a seven day trial of InDesign 2025 and try it.
I'm working on a form in InDesign CC 2025 right now, as in literally "I alt-tabbed over to this discussion after finalizing a Tamil document set in Nirmala UI." It works just fine, with the right preparations (making a paragraph style, choosing the World-Ready Composer, making sure the language is set to Tamil).
@lwalper if you really do want to know about "what was used prior to CS6 (or whatever is current) before even CS3" then I can tell you all about the history of pre-Unicode complex script publishing. However, I think you're probably more interested in finishing this project than in learning about the weird workarounds we used in the Pagemaker era. But if you are more interested in finishing the project than learning about what we did in the pre-Unicode era, I think Bob has your answer.