Apple A1243 keyboard no longer detected after using A1843 (MacBook Pro 2019 Intel)

Hardware:

· MacBook Pro 2019 Intel i5 Touch Bar (T2 chip) running macOS Sequoia 15.7

· MacBook Pro M1 running macOS Sequoia 15.7 (for testing)

· Magic Keyboard A1843 (recent)

· Apple Keyboard A1243 (older wired aluminum model)

· USB-C to USB-A adapter (Apple / hub)

Issue description:

I used my old A1243 keyboard on this MacBook Pro for years without any issue. A few months ago, I bought an A1843 Magic Keyboard to go wireless. Recently, I wanted to switch back to the A1243, but it is no longer detected at all — it doesn't show up in System Information > USB, and there's no reaction when plugged in.

What I've tried (unsuccessfully):

· SMC and NVRAM reset

· Full disk erase and clean install of the original factory version (Mojave)

· Tested with two different adapters (Apple and xTremeMac)

What does work (to isolate the issue):

· ✅ The A1243 keyboard works perfectly on a MacBook Pro M1 (same macOS Sequoia 15.3) using the same adapter

· ✅ The USB-C ports on my Intel Mac detect other devices (external drives, USB keys)

· ✅ The adapter is functional (tested with multiple devices)

· ✅ A Logitech keyboard is detected on the M1 Mac, but also not detected on my Intel Mac

Hypothesis:

It seems that since using the A1843 Magic Keyboard, my Intel MacBook Pro has "lost" the ability to recognize older keyboards (Apple A1243 and Logitech), while everything else works fine. It's as if the T2 chip or USB controller "learned" a new device type and no longer communicates properly with older ones.

Has anyone encountered this behavior before? Any ideas for a fix?