CAX30 : MTU adjustments have no impact, and resets to 1500 when rebooted

CAX30, without the S.

Firmware Version V1.4.12.2

On Spectrum cable in Western New York State.  I own it.  I did not lease it from Spectrum.

One of the reasons for ownership is so I can reduce MTU myself.  I have done that, in order to improve my work-from-home network performance.

Unfortunately, I recently noticed that reducing the value has no impact on my ping fragmentation tests(packets that are too large still pass with no frag), and the value resets to 1500 on reboot, regardless of what it was set to before.  None of the other settings reset like this.

Have any other users encountered this before?

My current guesses are:

1) Spectrum acquired low-level access to my CAX30 when they hooked it up, and are enforcing 1500 MTU.

2) My CAX30 is prioritizing suggestions from Spectrum, or perhaps the other devices connected to it, over my intended MTU setting.

I plan to open a Spectrum support request for 1), although they will just say "we can't change your MTU unless you lease a modem from us".

I'm here asking about the likelihood of 2).  Hopefully I won't have to pay for a support contract before it gets confirmed or denied here.

Thanks in advance for any insight.