I just got that same letter from the US embassy asking for a passport scan and will update when I get their response. They say it could take up to 72 hours. But isn't Chiang Mai immigration office closed starting on March 25?
TLDR : Answer Summary
The Chiang Mai immigration office is expected to close starting March 25, but the airport location will remain open. The situation is currently unclear as there are inquiries about the specific operations of the office during this time.