Bengali conjuncts with U+09A4 (Ta)

[Andreas Prilop]

To obtain the Bengali conjunct (ligature) "tka",
I write
        ta   virama   ka
      U+09A4 U+09CD U+0995

This worked fine in Windows XP but it no longer works
with the fonts Shonar Bangla and Vrinda in Windows 7.

Is there an explanation?