Own entropy seed generation does not work for <24 words/64 bytes

Seedtool doesn't calculate the seed/mnemonic from hex raw entropy smaller than 64 bytes (24 words). 
Test case from _Mastering Bitcoin Chapter 5 BIP39 table_ doesn't work for the 12 word case. For the 24 word case it works like it should. All tried cases <64 byte had the same problem.
_BIP39 Ian Coleman's Mnemonic Code Converter_ works for these cases. 