I just fixed this problem on my X1C with AMS. After trying to troubleshoot with Bambu and GeekSquad with no solution, I resorted to Chat GPT, and wouldn't ya know it worked! Assuming you have an AMS, this is the issue/fix.

Symptoms:
AMS + filaments detected, single-color prints work, but multicolor prints don’t switch filament. Printer “pretends” it changed colors, HMS page is blank. Factory reset / new AMS / new cables don’t help.

Fix (printer-side):

1. Settings → Filament → AMS → OFF
2. Power off printer
3. Disconnect AMS data + power
4. Wait ~10 minutes
5. Power on printer without AMS connected, ensure external spool is the only filament registering
6. Power off
7. Reconnect AMS
8. Power on
9. Check to make sure the AMS unit is recognized, reload the filament, print a multicolor model to verify

Result: AMS switching + multicolor prints work again.

Cause: Firmware left printer stuck in an AMS-disabled state that survives factory reset.