On my line the HG612 tends to bitswap more than the Zyxel VMG8924-B10A.
I noticed the HG612 tends to bitswap back into use the previously bitswapped deallocated upper tones whereas the Zyxel VMG8924-B10A tends to mark tones as unused and not bitswap them back into use when conditions change. I wasn't sure how to word this lol.
I noticed this since I tend to get increased noise at night and the D3 band is obliterated, the Zyxel tends not to bitswap and not use/reuse this band, whereas the HG612 I noticed it starts allocating bits again in the D3 band when noise levels improve.
Also I have no idea why the HG612 gives me the highest upstream sync.
I was able to get back fastpath on the HG612 since I got far less errors.
So basically on my line the Zyxel loves to push the downstream sync to the edge of stability at the cost of more errors and interleaving.
HG612 has aggressive bitswapping with a conservative sync.
Your mileage may vary.