I just trained a model on my own corpus. It has space group numbers and I replaced them with 'Xx1, Xx2,...., Xx229, Xx230' to avoid overlapping with some element names. But when I tried to get final embeddings from the model it says, some space group numbers (Xx105, Xx139 etc.) are `not in vocabulary` independent of the frequency! Why is this happening? I've tried to look up the code and couldn't figure it out.