Can confirm. See screenshots:
1) The setup, a 3x3 cube of stone with TNT placed at the center
2) The 1.7.10 results (upon lighting TNT, all stone blocks destroyed)
3) The latest snapshot results (upon lighting TNT, several stone blocks left, especially corners)
I've tried this multiple times and the results are consistent; TNT is definitely nerfed.
This bug is a fundamental flaw that nerfs all TNT mining and excavation in 1.8 Survival, I do hope it's not overlooked.
alexander- how was this exploited by players on your server to farm massive amounts of tnt? It would seem that this issue limits farming rather than boosts it.