Losing Tamil font going from windows7 to macOS

Hello,
I’m facing an extremely weird problem.
I have windows 7 on virtualbox on my mac.
Libre office is installed both on the host and virtual system (the most current version of lo).
Tamil text is displayed fine on the windows version.
But when I save it and open it in macOS libreoffice, I get garbage - Latin alphabet with diacritics.
I am using the same font on both oses (STMZ - 62).
With the correct keyboard, I’m able to type Tamil (with the same font) on the mac.
While I suspect this is NOT a LO problem, I’d be grateful for any help.
Re-entry is not an option, it’s a 250 page book.
Thank you for any help, guidance or pointers.
Ashok