LR13 - when an image is converted to BW, the color mixer tab does not always change to BW

When I edit a raw image that has been displayed in camera and taken as a BW image, the color mixer does not convert to BW. When I open this color mixer tab it says "monochrome profile applied" and does not show any of the color sliders expected in the BW tab. If I convert back the image to colors, the correct color mixer tab is available. From there, going back to BW, does solve the issue.

This issue seems to be camera specific.

This is true using pictures taken with a Sony A7RIV: it is configured to display images in camera in BW, and this setting is imported into LR, i.e. the image is displayed in BW, but the BW tab is not available.

It is however not the case with a Ricoh GRIIIX. When the in camera display is set to BW, the image imported in LR does not keep the setting and is displayed in color. In this case, after the conversion to monochrome, the color mixer tab is correctly modified to BW.

Am I doing anything wrong, or is this a bug?