Arris S33 upload stuck at 5 Mbps — provisioning reports "could not provision a valid bootfile"

I own an Arris SURFboard S33 and I am on 1 Gig Plan. Upload was previously around 70 Mbps. After a recent plan change and new equipment it dropped to 5 Mbps and has stayed there. Download is unaffected and tests at expected speeds. Tested with a PC wired directly to the modem, with my router and all other network equipment disconnected. Upload still measured 5 Mbps. This rules out any equipment on my side of the modem.

Upstream signal (from 192.168.100.1):

All four channels locked, SC-QAM, 6.4 MHz ltr">

• Channel 1 — 16.4 MHz — 36.8 dBmV
• Channel 2 — 22.8 MHz — 37.0 dBmV
• Channel 3 — 29.2 MHz — 37.3 dBmV
• Channel 4 — 35.6 MHz — 37.8 dBmV

Power levels are well within spec. No OFDMA upstream channel present, and all frequencies are below 42 MHz.

What support has already tried:

• Multiple reprovisioning attempts over chat
• Their system reports the status as "The modem could not provision a valid bootfile"
• A modem reboot was eventually pushed; the modem re-requested its config and upload remained at 5 Mbps
• Signal levels were confirmed in spec
• I was ultimately told there is no issue with the Xfinity settings and to contact the device manufacturer

My questions:

1. My account is provisioned for up to 35 Mbps upload, but I am only receiving 5. Why is there a gap between the provisioned rate and the delivered rate?
2. Why is the bootfile reporting as invalid? Is the system attempting to push a config the S33 cannot accept, causing it to fall back to a minimal default?
3. Can I be provisioned with a config the S33 will accept so I can reach the 35 Mbps I'm provisioned for? Four bonded SC-QAM channels should deliver well above 5 Mbps.

I understand the S33 is not on the approved list for mid-split enhanced upload speeds. I am not asking for Next Gen speeds. I am asking why I am at 5 Mbps when the standard rate for this modem and signal quality should be considerably higher.