Censys expands queries to provide additional functionality. For example, the query a:zmap.io
executes a DNS lookup and expands to ip:22.214.171.124.
Your query was changed from:
(autonomous_system.organization: (hospital or clinic or medical or healthcare)) AND location.country: "Vietnam"
(autonomous_system.organization: (hospital OR clinic OR medical OR healthcare)) AND location.country: "Vietnam"
If this isn't what you intended, you can run your raw query without expansion.