new info:
run twice confirm the issues.
Watch the netns change, and find:
The netns qrouter* missed at:
qrouter-d7faa15d-6f88-44de-8fef-7de54e7fee88 (id: 1)               qdhcp-5de5301b-a3f4-482d-8a1b-23badaff80c4 (id: 0)
Thu Feb 20 07:37:21 GMT 2020
2
qrouter-d7faa15d-6f88-44de-8fef-7de54e7fee88 (id: 1)
qdhcp-5de5301b-a3f4-482d-8a1b-23badaff80c4 (id: 0)                 Thu Feb 20 07:37:22 GMT 2020                                       1

07:37:21

And that time the l3-agent had already tried for iptables restore many times.
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager line 2: CHAIN_ADD failed (Device or resource busy): chain OUTPUT
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager line 12: RULE_INSERT failed (No such file or directory): rule in chain OUTPUT
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager line 13: RULE_INSERT failed (No such file or directory): rule in chain POSTROUTING
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager line 14: RULE_APPEND failed (No such file or directory): rule in chain POSTROUTING
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager line 15: RULE_INSERT failed (No such file or directory): rule in chain PREROUTING
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager
2020-02-20 07:37:21.735 12 ERROR neutron.agent.linux.iptables_manager
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info [-] Failure applying iptables rules: IpTablesApplyException: Failure applying iptables rules
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info Traceback (most recent call last):
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info   File "/var/lib/kolla/venv/lib/python2.7/site-packages/neutron/common/utils.py", line 158, in call
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info     return func(*args, **kwargs)
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info   File "/var/lib/kolla/venv/lib/python2.7/site-packages/neutron/agent/l3/router_info.py", line 1189, in process
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info     self.process_address_scope()
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info   File "/var/lib/kolla/venv/lib/python2.7/site-packages/neutron/agent/l3/router_info.py", line 1152, in process_address_scope
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info     self.process_floating_ip_address_scope_rules()
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info   File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info     self.gen.next()
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info   File "/var/lib/kolla/venv/lib/python2.7/site-packages/neutron/agent/linux/iptables_manager.py", line 438, in defer_apply
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info     raise l3_exc.IpTablesApplyException(msg)
2020-02-20 07:37:21.736 12 ERROR neutron.agent.l3.router_info IpTablesApplyException: Failure applying iptables rules
2020-02-20 07:37:21.738 12 WARNING neutron.agent.l3.agent [-] Hit retry limit with router update for d7faa15d-6f88-44de-8fef-7de54e7fee88, action 3

The last error message here for l3-agent is also 07:37:21

==================================================================
After that the router namespace disappear.

So the root cause is with l3-agent service.

After reboot, it recover and netns qrouter* come back.