Convert to Grayscale preflight turns photos very dark

We have certain items with grayscale photos that use CMYK ink that we need to convert to black ink only. However when I use Convert to Grayscale or Digital Printing (B/W) to achieve this, the photos become extremely dark, unless I turn on Output Preview, at which point they appear brighter.  The dark appearance happens whether I’m viewing them in Acrobat or a web browser.  Why is this happening, and how can I prevent it?  It has caused confusion when proofing to clients.

Using Print Production → Convert Colors → Gray Gamma 1.8/2.2 gives slightly better results for the photos but turns all of the text a faded gray color.

Examples shown below: