Is muscle loss a concern while losing weight on a GLP-1 medication, and does that change what you should be eating?

Yes, it's a legitimate concern. Research on GLP-1 therapy suggests a real portion of total weight lost (estimates vary, but commonly cited in the range of roughly a quarter to a third or more) can come from lean mass rather than fat alone. Because of this, many clinicians now recommend prioritizing adequate protein intake and resistance exercise specifically to help preserve muscle during treatment, rather than just eating less of everything across the board. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/