I shot some interviews where, before shooting, I zoomed in on the eyes and had them perfectly focused. I zoomed back out....but when I watched the footage later, it was a bit soft. Super irksome. Is it possible the focal length changes a bit as I zoom out? That would explain it.
It's not the focal length changing, but the focus distance--and yes, it's likely changing. As far as I know, the lenses of the XA-series are not parfocal. It's better to set focus using the focal length you're going to use, and then lock it off in Manual Focus mode.