Would not surprise me if a DOCSIS modem driver bug could be causing this sort of issue, fiber router/modems like the Q1000K from AT&T/Centurylink/Quantum Fiber have all sorts of crazy driver bugs[0]. Some of which I know can cause packet corruption and accidental TCP session tracking due to poorly designed bridge driver code.
[0] https://github.com/jameshilliard/q1000k-hsgmii-pad