"I was able to see on a list of possible side effects for Kevzara, that it can exacerbate side effects from pre-existing auto immune disorders? So my question is, has anyone else had this happen?"
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No but I'm an Actemra user. I seem to get outnumbered by the Kevzara users for PMR so I will defer to them. However, Actemra and Kevzara have the same classification because they are both IL-6 inhibitors.

I have multiple autoimmune disorders and Actemra seems to control all of them. I don't think any of my pre-existing autoimmune disorders before PMR were exacerbated. I do believe that Kevzara and Actemra only work for certain autoimmune disorders that primarily involve IL-6 related inflammation and not other autoimmune disorders that don't primarily involve IL-6 inflammation.

There are many other autoimmune disorders that involve different inflammation pathways and different tissues in the body so they all can't be treated the same.

The universal anti-inflammatory medication is probably prednisone which works for nearly every autoimmune condition. Prednisone works for almost all autoimmune conditions because it doesn't target just one specific pathway. Prednisone is more like a "master switch" that turns off the immune system by inhibiting multiple inflammatory pathways.

One reason Prednisone comes with so many side effects is because it replaces the hormone cortisol. Cortisol is regulated naturally by the body because too much cortisol is just as bad as too little cortisol.

When we take Prednisone instead of cortisol we almost always are taking too much or too little prednisone. The body doesn't have any way to regulate the amount of prednisone we take. The side effects from Prednisone is the only means our bodies have to tell us if we are taking too much or too little Prednisone.

Overall, my side effects are minimal from Actemra compared to the side effects I had from Prednisone. That doesn't mean there aren't any potential side effects from IL-6 inhibitors.\
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By the way ... I have trigeminal neuralgia (TN) instead of occipital neuralgia (ON). I had flares of TN whenever my systemic inflammation levels from PMR were too high. I used to call TN my "inflammation alarm." I needed surgery to stop TN.
https://www.mayoclinic.org/diseases-conditions/trigeminal-neuralgia/symptoms-causes/syc-20353344
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Do you have GCA that is causing you to have occipital neuralgia? Occipital neuralgia in giant cell arteritis (GCA) is frequently caused by vasculitis and swelling of the occipital artery. This inflamed artery irritates the adjacent nerves, leading to continuous aching or burning pain at the base of the skull.
https://pmc.ncbi.nlm.nih.gov/articles/PMC11729197/