It’s quite possible that some layer colors in LightBurn simply don’t exist in AutoCAD’s color set, so when exported, LightBurn finds the closest match. When re-imported, it does the reverse. That would explain why C23 / C24 both come back in as C24 - they’re probably so close that they map to the same export color in DXF format.
And the tool layers (T1 and T2) will no longer be imported into, as a number of users complained about this, so they were removed from the import mapping list.