This is normal.

While the 68K Family in general supports MMU.
Each 68k MMU works differently and needs be programmed in its "own" way.

As you will know a MMU is not meant to be used/programmed by any software or program. The MMU is a tool that only the operating system is allowed to use.

As the MMU is used for managing Memory,
the Design of MMU changed the same way a Memory Chips changed many times over the year.

This means the the MMU of the 68030 made after some years not much sense .. and the 68040 has therefore a different MMU.
Of course also the design of the 68040 made after some years not much sense ... And today a useful MMU would be done differently of course.

You will recall the first Amiga was first sold with 0.25 MB only ...
This was quickly doubled to 0.5 MB ..
And later Amigas had 1 MB or 2MB and you could of course upgrade them.

Today our Amiga comes with 512 MB..
Maybe we will have models with 2028 MB at some point.

I think its very easy to understand that
an MMU which was designed to manage 1-8 MB
might do a very bad job when you have 100 MB or 1000 MB memory.

As you know AMIGA OS does not use MMU.

Its normal that SYSINFO by itself will not see /find a new MMU.

Having now spent some time exploring Coffin and a super-charged A600, I have some experience to report. A brief description of the configuration:

A600 (Full recap November 2022) with 1Mb trapdoor expansion.
Manticore running alpha core revision 9662.
I have two power supplies -- the one which was originally shipped with the A600 and a third-party 60W PSU from eBay, both of which have been tried.
32Gb CF (containing latest Coffin build) and 32Gb SD card (empty).
Display is LG CX 65" OLED TV (2020).

The system is not stable, and crashes intermittently.  Sometimes it can be reset by the keyboard, other times the PSU needs to be power cycled.  The crashes are random and I cannot reliably reproduce them.

On the display, some display modes extend beyond the viewport of the screen.  On 640x480, the top left and bottom right-hand corners are off-screen. 800x600, 1024x768 work perfectly.  Amiga modes are all displayed using the full bounds of the display, centered with no clipping.  Amiga demos are sooo nice on an OLED.

I am not too concerned about the display, but some direction on what to do about the instability would be gratefully received.  Thanks!