When Export's Image Format is set to JPEG XL, the Export window doesn't remember that the previous setting of Bit Depth was 16 bits.
Tested on LR 13.5.1 / Windows 11, Mac OS 14.6.1:
1. Reset LR's preferences.
2. Select a raw file and do File > Export.
3. In File Settings, set Image Format to JPEG XL and Bit Depth to 16 bits:

4. Click Done.
5. Do File > Export, and observe that Bit Depth has been reset to 8 bits/component (incorrect):

6. Change Bit Depth to 16 bits/component and click Export.
7. Do File > Export and observe that Bit Depth has again been reset to 8 bits/component (incorrect).
8. Change Bit Depth to 16 and click Export.
9. Do File > Export With Previous and observe that file size of the export from step 8 is quite a bit larger than that from this step. Use the "jxlinfo" command (on Mac, "sudo port install libjxl") to observe that the step 8 .jxl is 16-bit while the step 9.jxl is 8-bit (incorrect).