If this article is true, a mutation is a new virus capable of infecting anew.
A mutation is simply some variation; not necessarily good for us or bad for us on its face and by no means necessarily impacting immunity in any way. Most mutations are minor and don’t change much. The mutation that happened earlier this year with SARS-CoV-2 enabled it to more efficiently enter human cells, which made it more infectious and allowed that strain to become dominant in most places. But it had zero impact on immunity.
a mutation is not a new virus, its the same virus that has just mutated some. Immunity wouldn’t be lost.