GLP-1 reduces calorie intake and puts many people on a deficit (typically on purpose). This of course will reduce muscle just like any other calorie deficit anyone runs long term.
When you are losing 5% of your bodyweight per month (as I was, and many do) a substantial portion of that is simply going to be mean muscle mass. You can counteract some, but not all, of this by heavy resistance training. It's very difficult to not lose muscle mass while losing weight - it takes extreme measures for most folks (e.g. athletes) to do so.