0xDEFE should be udf #0xfe since document DDI0406C.a which declares that it's the first to define an assembler mnemonic for this encoding. Capstone currently emits trap.

Minimal code to reproduce:

#!/usr/bin/env python
from capstone import *
md = Cs(CS_ARCH_ARM, CS_MODE_BIG_ENDIAN|CS_MODE_THUMB)
for i in md.disasm("\xDE\xFE", 0):
	print("0x%x:\t%s\t%s" %(i.address, i.mnemonic, i.op_str))

Bit fields are:

+----+----+--------+
|1101|1110|  imm8.8|
+----+----+--------+
|1101|1110|11111110|
+----+----+--------+

Online disassembler supports this claim.