Found the solution here: Https suddenly missing from webhook and callback URLs - #4 by krisn0x
I stripped SERVICE_URL_N8N_5678 AND WEBHOOK_URL from compose and used these env vars:
N8N_HOST=n8n.redacted
N8N_PORT=5678
N8N_PROTOCOL=https
N8N_EDITOR_BASE_URL=https://n8n.redacted
WEBHOOK_URL=https://n8n.redacted