rubber membrane used for chimney crown

[Sam Morris]

I'm sure this material is not approved for the chimney crown, maybe the roof.

[Bob Harper]

Not approved. Often used where there is a significant water intrusion problem and this is a stop gap to buy a few days until the contractor can restore the crown. If that is a solid fuel liner then you do have a much higher risk of fire.

[Matthew Steger]

I'm sure this material is not approved for the chimney crown, maybe the roof.
I would say it is likely not approved for that install and should be removed (and the crown properly repaired if damaged underneath) by a qualified chimney professional.