Large HTTPS uploads to Cloudflare fail on my connection; please escalate to network engineering

Hi,

Since at least September 22, large uploads from my home connection to any site hosted on Cloudflare fail mid-transfer. I've isolated it to the route between my Xfinity connection and Cloudflare, and I'd like it escalated to your network team. A modem reset or speed test won't show it, because downloads and small uploads are fine.

What fails

• HTTPS uploads larger than about 100–500 KB to Cloudflare-hosted servers (AS13335) are reset mid-stream. A 5 MB upload to speed.cloudflare.com fails 10 out of 10 times, and uploads to api.anthropic.com (also behind Cloudflare, Newark/EWR) fail the same way.
• A packet capture shows my machine sends the data cleanly with no retransmissions. The Cloudflare server then sends a TLS fatal alert and resets the connection. That pattern usually means the data arrived altered in transit.

What works (same computer, same time)

• Uploads to non-Cloudflare networks (Amazon, Google, Fly.io): pass.
• Downloads from Cloudflare: pass.
• The same Cloudflare uploads over a mobile hotspot: 16 of 16 pass.
• The same uploads over a VPN on my home Xfinity connection: 10 of 10 pass.

Already ruled out

• My computer and its software (works on the hotspot).
• MTU/packet size (1500 works end to end), and upload speed.
• My own equipment's settings: I tested with its optional security features off one at a time, saw no change, and restored them all.
• The modem's cable event log shows no T3/T4 timeouts or ranging errors.

Route to Cloudflare (Xfinity network hops; both failing destinations take the same path) 3 68.87.156.237 4 69.139.221.134 5 68.85.186.5 6 96.110.42.13 / 96.110.42.1 7 96.110.42.5 8-9 * (no reply) 10 162.158.61.103 (Cloudflare, Newark)

Separate issue: my modem gets no IPv6 prefix delegation, so my network has no IPv6.

Could you check the path from my connection to Cloudflare (around 96.110.42.5 and the handoff to 162.158.61.103) for a faulty link or device, and look into the missing IPv6 prefix? I can share account details in a private message.

Thanks