Thanks for your reply.
I would suggest a repeat MRI of lumbar sacral spine to confirm removal of compression.
If the spondylolisthesis was not fixed persisting instability can also be responsible for your symptoms.
In cases of severe stenosis at times the changes in the nerves due to stenosis become permanent and do not allow improvement in symptoms after decompression.
In my opinion it is either of the two possibilities.
Feel free to ask any follow up questions.
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I understand that this may not be the answer you wanted or expected to get but I have to be truthful as anything less would be unethical.

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