Okay, so this is borked (but not your fault, we do have an issue currentky with cakes auto-auto-overhead-adjustments), you also have GRO/GSO enabled. But
The "maxpacket 1492" fits with what one expects on a pppoe interface, so if you would add 8 bytes you and up with 1500 which certainly is too little, you will almost certainly need at least 18 bytes for ethernet (6 dstMAC, 6 srcMAC, 2 ethertype, 4 frame check sequence) and 4 bytes for VDSL2/PTM; potentially another 4 to 8 bytes for single or double VLAN tags. My ISP currently uses 1526 frames, so on a pppoe interface (where the PPPoE header is not yet added) I actually need to request 1526-1492 = 34 bytes of overhead to properly describe the link to sqm.
Now there is a bigger issue with current cake visible in:
So cake will add variable overhead for ingress and egress. So I would try to add the following to the advanced option strings: "raw overhead 34 mpu 64", this should sidestep cake's broken auto-auto-adjustments and still properly describe the link.
Why does this not directly show up in tests? Well, bandwidth and overhead are not independent variables, specifying too much overhead effectively reduces the bandwidth and setting the bandwidth too low allows for too small an overhead. Since one often really does not know the exact numbers for neither but empirically tries to get a working combination it simply is not guaranteed that the set one ends up with truly describes the links limits well or just happen to work well-enough.
In case you ask yourself, yes the cake developer is working on fixing the auto-auto-adjusments making things simpler in the future but no ETA yet.