When receiving Respawn (0x33, client-bound) packets from a Spigot 1.10 server, these are encoded incorrectly. The packet format should be as follows:
4 bytes Int: dimension (-1:nether, 0:overworld, 1:end)
1 byte Char: difficulty (0:peaceful, 1:easy, 2:normal, 3:hard, bit 3 (+8): hardcore)
1 byte Char: gamemode (0:survival, 1:creative, 2:adventure, 3:spectator)

Examples for a Respawn packet received from various servers:

• from a Spigot 1.10 server (64.34.190.101:28965):
  00 00 00 0b 00 01 07 64 65 66 61 75 6c 74 .......default
  dimension=11 (???), difficulty=peaceful, gamemode=creative

• from a Vanilla 1.10 server (locally installed):
  ff ff ff ff 01 01 04 66 6c 61 74 .......flat
  dimension=-1 (nether), difficulty=1 (easy), gamemode=1 (creative)

00 00 00 00 01 01 04 66 6c 61 74 .......flat
dimension=0 (overworld), difficulty=1 (easy), gamemode=1 (creative)

• from a Waterfall 1.8/1.9/1.10 server (2b2t.org):
  00 00 00 01 00 00 07 64 65 66 61 75 6c 74 .......default
  dimension=1 (end), difficulty=0 (peaceful), gamemode=0 (survival)

ff ff ff ff 00 00 07 64 65 66 61 75 6c 74 .......default
dimension=-1 (nether), difficulty=0 (peaceful), gamemode=0 (survival)

ff ff ff ff 03 00 07 64 65 66 61 75 6c 74 .......default
dimension=-1 (nether), difficulty=3 (hard), gamemode=0 (survival)

My guess is that Spigot 1.10 sends the values of dimension,difficulty,gamemode as shorts, which would interpret the output from the first example as dimension=0 (overworld), difficulty=11 (hard+hardcore), gamemode=0 (survival), while it should encode them as int,byte,byte (in both cases followed by a string)