The IPv6 neigh sometimes entry into failed status and could not send package to the destination

1. My network topology as below, the pod node pull file from ftpserver as sftpclient every 5 minutes;
   

2. In normal cases, the pod learns the MAC address of the gateway routing node(2409:802f:5a06:120f::1009:181) through NDP NS packets, as shown in the following figure:
   

3. The ip -6 route list config as below:
   

4. My problem is that:
   The system intermittently fails to pull data from sftpserver. The cause is that the MAC address of the gateway node cannot be get by NDP NS message. According to the packet capture result of Wireshark, the SFTP tcp message does not trigger the NS message. The ip -6 neigh show command shows the neigh to the gateway node(2409:802f:5a06:120f::1009:181) status is failed. The error "No route to host" is reported on the SFTP application side, as shown in the following figure.

In normal cases, NS packets are triggered, as shown in the following figure：

When the problem occurs, the service side reports the following error.The ip -6 neigh show command shows the neigh to the gateway node(2409:802f:5a06:120f::1009:181) status is failed:

What could be the reason for this problem?