I'm seeing these messages nonstop in the system logs on my RT-AX88U;
Apr 10 06:32:25 kernel: nf_conntrack: expectation table full
Apr 10 06:32:25 kernel: nf_conntrack: expectation table full
Apr 10 06:32:25 kernel: nf_conntrack: expectation table full
Apr 10 06:32:25 kernel: nf_conntrack: expectation table full
Apr 10 06:32:25 kernel: nf_conntrack: expectation table full
Apr 10 06:32:25 kernel: nf_conntrack: expectation table full
Apr 10 06:32:26 kernel: nf_conntrack: expectation table full
Apr 10 06:32:26 kernel: nf_conntrack: expectation table full
Apr 10 06:32:26 kernel: nf_conntrack: expectation table full
Apr 10 06:32:26 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: net_ratelimit: 7 callbacks suppressed
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
Apr 10 06:59:03 kernel: nf_conntrack: expectation table full
I did find thread;
https://www.snbforums.com/threads/nf_conntrack-expectation-table-full-and-other-log-oddities.55415/
So I tried to limit TCP Connections from the default 300000 down to 28244 as the recommended value seemed to be your ct_hashsize * 4 and the hashsize is 7061 as default on my RT-AX88U. I did also run echo 152 > /proc/sys/net/netfilter/nf_conntrack_expect_max.
The systemlogs was clear for a few hours until the same messages got back.