Yes—on the Yashica Mat/124G, haze can be internal and may not be removable with normal cleaning. According to the community answers, two likely causes are:

1. Cement deterioration in a glued rear doublet. If the haze is in the cement layer, it generally cannot be cleaned off.
2. Fungus residue or etching on glass surfaces. Even if the fungus is removed, etched glass will still look hazy; fixing that would require repolishing, which is rarely practical.

One answer also notes that the front two elements are not cemented and have an air gap, so fungus or haze can exist between them. Those elements can sometimes be separated for cleaning, but it is tricky and risks damage.

So: yes, internal haze is possible. If it is between the front air-spaced elements, careful disassembly may allow cleaning. If it is in a cemented group or the glass is etched, additional solvents like acetone are unlikely to help and may create more risk than benefit. The realistic options are careful service, living with the haze if image quality is acceptable, or replacing the affected lens/group.