It may be a pretty weird issue, but I can't think of a cause for the rings in the textures crimson and warped stems.
The thing is that in real life rings are caused in trees by the annual temperature change (winter - summer). Fungi are not trees and do not have these rings at all.
Thinking of the nether itself: the Nether has no winter or summer either, which does not cause rings at all. The crimson and warped fungi grow in the nether where they don't have to make it through the cold and hot periods of winter and summer: there are no seasons in the nether...
So why do warped and crimson fungi have rings? It's pretty weird if you realise what these rings actually are: they now look like wood logs and not like hyphae logs...
If the textures are modified a bit, they could look much more like fungus stems: