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.