Pear,
great to hear that you are using
https://www.map.army.
Regarding your question: Yes, this is by design. The style of the coordinate grid is not stored in the .milx files, so it isn't preserved between sessions. When the app launches, the grid always starts at maximum opacity, and it only picks up your configured value once the opacity slider in Settings is touched.
We'll consider persisting the grid style in a future update. In the meantime, adjusting the slider after launch will restore your preferred opacity.