The documentation for zabbix_sender says "If target item has triggers referencing it, all timestamps must be in an increasing order, otherwise event calculation will not be correct" here:
https://www.zabbix.com/documentation/current/en/manpages/zabbix_sender

It really shouldn't accept data that is older than the newest value for an item. At least not if this creates issues with triggers, notifications, etc. It's bound to create problems that are difficult to diagnose.