Believe the report
Start from the customer saw something real. Thank them and say you are looking into it.
Ask for the missing details
Request steps, browser or device, the account affected and a screenshot or recording.
Try their conditions
Test with the same plan, role and data where you can. Many bugs depend on settings, not code.
Tell them what you tried
Say what you tested and what you saw so they can correct a wrong assumption.
Keep it open with a date
Promise a next update and keep a known-issue note so a second report links to the first.