I had to apply moderate heat, using a hair dryer, then unscrew very hard using two pliers.
Once opened, I replaced the stock optic with a 20° Shamian (frosted) from Yajiamei Optics, like I did to successfully improve the beam of my Skilhunt E3A.
No more rings, but although I don't have a way to measure it, I notice a noticeable decrease in brightness WRT the original optic, both in direct lighting and in a ceiling bounce test.
A piece of DC film on the original optic gives as good a result on the beam, without any noticeable brightness loss.
Strangely I had not noticed such a dimming effect on the E3A with the Yajiamei optic, but maybe I did not compare both options carefully enough.