Automatically disconnect 1 pppoe connection after about 30 minutes

Hi everyone!
I am using a mikrotik ccr2004-16g-2s+ device (OS 7.15). I got a problem when I configured LBC PPC for 2 internet lines. After the configuration was completed, the device worked for about 30 minutes then ISP1 or ISP2 internet lines lost connection.

With these same two internet lines, ISP1 and ISP2, I use another Mikrotik device with OS 6.48 and still have the same configuration, it works very well. At the same time, I tried using other devices such as draytek and pfsense, the two internet lines here still worked well with the loadbalancing function on them.

When I brought this mikrotik ccr2004-16g-2s+ (OS 7.15) device to another place with 2 other ISPs (ISP and ISP3), the device worked normally, I just changed the pppoe account and kept the configuration

I really don’t understand why the mikrotik ccr2004-16g-2s+ (OS 7.15) device has a problem disconnecting one of the two network lines.

please help me with this issue