Can a prolonged chalazion cause long-term astigmatism in adults?

It is possible for a chalzion to cause  astigmatism due to pressure on the cornea. Not all chalazia will resolve with conservative treatment. After five months much of the inflammatory component may have turned to solid scar. Excision may be the only option if you want it resolved. You should consult with your Ophthalmologist or an Oculoplastic surgeon.

Yes, Chalazion can cause astigmatism if it is of a large size , especially on the upper eyelids. They can change the shape of cornea. But, it is reversable once removed.