Inspect the Dimension Properties:

Double-click on the first dimension to open the text editor. Notice the dark solid color background behind the dimension text.

This indicates that a background mask is enabled for this dimension.

Check the Background Mask Settings:

Select the dimension and go to the Properties palette.

Under the 'Text' section, find the 'Fill Color' or 'Background Mask' setting. Ensure it is set to an appropriate color (e.g., the same as the background).

Compare with the Second Dimension:

Double-click on the second dimension and observe the absence of a background color during editing.

In the Properties palette, confirm that the 'Background Mask' is either disabled or set to 'None.'

Adjust Settings if Needed:

If you want both dimensions to have a background mask, enable the 'Background Mask' for the second dimension and choose the desired color.

AutoCAD Documentation on dimension styles and background masks: Dimension Styles