After stacking two switches, it was observed that the outbound traffic is running over a single link without load balancing.
this is the reason
Here's the English translation of the paragraph about Local Preferential Forwarding:
Local Preferential Forwarding (specific to clustering/stacking systems): In stack/cluster systems, Huawei switches enable "Local Preferential Forwarding" by default. This means that traffic entering from a particular member switch will preferentially egress from that same switch's local interfaces, avoiding forwarding across devices. This significantly improves convergence performance during cluster failover, but at the cost of potentially sacrificing some load balancing effectiveness. If higher load balancing is required, this feature can be disabled using the ecmp local-preference disable command.
https://support.huawei.com/hedex/hdx.do?docid=EDOC1100407960\&id=ZH-CN_CLIREF_0177116050
check the change
Display cur | in ecmp local-preference disable