Hi!
If you have any other problems with LDAC, even with using developper option, don't waste you time any further.
(I've spent so much time to correct the issue in this way)
Developer solution's won't work.
And you'll not have the choice to enable LDAC in Bluetooth normal user setting
Reboot your Xiaomi with hard reset.
Everything will become clear and operational.
LDAC Choice will appear correctly again.
Best regards