The problem is the reply path.
With ping -t
you will only change the TTL for the packets that you are sending.
The reply packets will have a starting TTL set to the OS default (64 in Linux).
So in the forth case your outgoing packets will have a TTL of 10 and a size of ~10000. The packets that are coming back form 10.6.7.10 will have a TTL of 64 and a size of ~10000 and thus will be dropped.
You can use a sniffer, for example wireshark, to verify this.