From the source:
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/src/libs/zbxpoller/checks_internal.c#440

else if (0 == strcmp(tmp, "maintenance")) /* zabbix["host",,"maintenance"] */
{
/* this item is always processed by server */ 

it's using hardcoded argument "maintenance".

If an argument "maintenance" is parsed using user macro AND the item key is configured behind a proxy, it starts to report an unreliable outcome.

Need to either:
1) Report reliable outcome (expand user macro before calculating status) or
2) Show a better error message that it's impossible to calculate the maintenance status behind a proxy. Ask the user to kindly use a static argument without user macro.