It would be nice to add notes to Zabbix official documentation about "maintenance" - https://www.zabbix.com/documentation/5.0/en/manual/maintenance that would describe some nuances regarding DST days.
For example, a user defines the maintenance period set to 1 day and this day is the DST day. The actual period of the maintenance is always 24 hours (as internally Zabbix calculates days in hours). However, these 24 hours may end on the 23rd hour of the DST day if we turn clocks back by 1 hour (in Autumn) or be prolonged to the end of the 1st hour of the next day (counting from DST day) if we turn clocks forward (in Spring).