Good Morning, @hawkone. I am a transplant recipient, and have never been prescribe Lactulose. I think that you are wise to check the safety of you medication before taking it.
I did find some information about storage.

Storage:
Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.
https://www.mayoclinic.org/drugs-supplements/lactulose-oral-route/proper-use/drg-20528356
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What should I know about storage and disposal of this medication?
Keep this medication in the container it came in, tightly closed, and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom).
https://medlineplus.gov/druginfo/meds/a682338.html
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I have not seen anything about expiration date, and that is another concern that I would have.
Hawkone, Have you contacted your pharmacist/or a local pharmacist about this?