I haven’t found that mindfulness caused my aphantasia, but I have found mindfulness to me impossible for me to do and triggering, in part due to my aphantasia. A lot of mindfulness says to simulate experiences, imagine places or feelings, or focus on something like breathing. I’ve found that I just can’t do any of the simulation, I have no imagery to work with, all I can do is think of the thing and nothing happens because I can’t make my senses experience things that aren’t actually happening to me right now. I also found the parts about ‘racing thoughts’ just didn’t apply due to my aphantasia, my brain doesn’t independently make thoughts. It’s just empty, I have to manually think everything or it’s just silent, and focussing on my breathing just took it off automatic and now I have to try to keep myself breathing. I used to have some wear imagery as a child, which I lost as I got older. Perhaps if your aphantasia developed desperately, mindfulness was what allowed you to recognise your aphantasia, if it didn’t cause it.

Given the number of people that practice mindfulness meditation and the number of people that have aphantasia I don't think any kind of link would be sensible to suggest, there would be many more times the number of aphantasics than we observe if that were the case. There are hundreds of millions of meditators that do not have aphantasia. Human beings are absolutely horrible at determining causation and proximate cause is something we tend to assume. Meaning your loss of visualization occurred around the time you started practicing mindfulness but that doesn't mean it was the cause, totally unrelated things happen at nearly the same time all the time with no reason to connect the two in a causal manner.

I wouldn't think mindfulness meditation would cause aphantasia. I dont think its anything we can do or not do that will create it. I think it is more the way the brain is wired.  But...I could be wrong.  I've done a lot of therapy to build neural pathways and overcome brain damage.  I can sometimes see colors in my mind and its not always cave black anymore. Sometimes I see random shapes and abstract things, so my experience is kind of reversed from yours.