PPPoE reconnect, why?

My PPPoE connection restarted and I found this in the system log:
No response to 5 echo-requests
Serial link appears to be disconnected.

However, on the WAN configuration page "LCP echo failure threshold" is set to 0 so it should not happen at all.

Is it a wrong configuration, a misunderstanding or a bug in OpenWrt?

I use this firmware: OpenWrt 19.07.7 r11306-c4a6851c72 / LuCI openwrt-19.07 branch git-21.044.30835-34e0d65

Thank you.

A line restart might be triggered by the ISP. Could that be the case?
What is the output of ps wwww | grep pppd ?

/usr/sbin/pppd nodetach ipparam wan ifname pppoe-wan lcp-echo-interval 1 lcp-echo-failure 5 lcp-echo-adaptive +ipv6 set AUTOIPV6=1 nodefaultroute usepeerdns maxfail 1 user xxxxxx password yyyyy ip-up-script /lib/netifd/ppp-up ipv6-up-script /lib/netifd/ppp6-up ip-down-script /lib/netifd/ppp-down ipv6-down-script /lib/netifd/ppp-down mtu 1492 mru 1492 plugin rp-pppoe.so nic-eth0.2
18561 root 1216 R grep pppd

This is the output. It says lcp-echo-interval 1 lcp-echo-failure 5 which is not consistent with LuCI settings.

Try to select another value, e.g 10, verify it has changed from the ps command, then make it again 0.

Thank you, I tried it. I changed it to 10, and yes, it changed. Then made it again to 0, and it changed back to lcp-echo-interval 1 and lcp-echo-failure 5. I don't think it should work this way.

Can you try to change the lcp-echo-failure to 0 and see if it occurs again?
In any case you can open a bug report.