InDesign and managing grayscale images

In fact, it's not exactly the original image that you're importing into InDesign but a preview... The eyedropper gives you the color of the screen and not the color of the image. If you have calibrated your screen accurately, you will be close but not exact...
Then there is the display which is modified by the output device simulation that you have configured in the "View/Proof Setup" menu.
And finally, another thing to take into consideration are the profiles that will correct or alter the PDF output afterwards.

You also need to define the working space for RGB and CMYK in the "Edit/Color" menu, as this will give coherence to your work across all Adobe software, or you can go through syncing and "Bridge."

Thank you very much for your response!

I agree with you that InDesign does a preview of the image. The color difference is already visible in InDesign but is still present in my production PDF. That's what surprises me!

Even though, for me, the eyedropper does not measure the color of my screen but the "real" color of the hue of an image (the measured pixel value will be similar on my computer/screen or that of another but would appear differently to the eye, depending on the calibration), there is still a noticeably measured difference. The eyedropper measures an area at 0% in Photoshop and that same area is at 20% black in my production PDF (measurement made with the “Output Preview” tool found in Acrobat's prepress). Whether my screen is calibrated or not, this difference exists. I have no problems of this type as soon as my images are in CMYK; they are in my production PDF as they appear in Photoshop. It is only with these same images in grayscale that Photoshop or InDesign adds 20% gray in the white areas. And perhaps in other shades of gray as well, but it's less obvious.

I think there is an option to uncheck or something somewhere. I have never had this problem before the new version of the Adobe suite and I have a lot of production files to my credit ;) By the way, since this 2023 version, Photoshop automatically handles the blacks in my images to a maximum ink coverage of 300%. I admit, it suits me! But this is something that Photoshop did not automatically manage before... an additional mystery for me ;)

Oh okay..
If you have a difference in your PDF, that's not normal...
So what you observe in Indesign (20% black instead of 0%) is reflected in your PDF, which means that during the import into Indesign you have a conversion...
Check your import settings..
But honestly, going from 0 to 20% is huge..! It’s like you have a layer hanging around that’s adding everywhere..
By the way, wouldn't you have the same variation in the grays? (+20%)

I also find that adding 20% gray is huge and strange. I double-checked and there's no "ghost" layer lingering in my Photoshop files.

But I managed to "work around" the problem: I export my images in grayscale as jpeg and the problem is solved. The other images were in tiff format. I don't know if the problem came from there and if so, why?

In any case, I have my images in grayscale as they should be, that's what matters to me.

Thank you in any case for your valuable help!