Gnome Panel becomes fully transparent when window is not maximised!

the behavior in your question is from the user-shell theme.

you should look for the below values in a style sheet that you are using as a user-shell theme.

#panel -->> when no window is maximized.
#panel.solid -->> when a window is maximized.

in below example I took /usr/share/gnome-shell/theme/ubuntu.css file in my Ubuntu 18.04

at line number 658

/* TOP BAR */
#panel {
  background-color: rgba(0, 0, 0, 0.2);
  /* transition from solid to transparent */
  transition-duration: 500ms;
  font-weight: bold;
  height: 1.86em; }

and at line number 1950

/* panel color */
#panel.solid {
  background-gradient-direction: vertical;
  background-gradient-start: #58554d;
  background-gradient-end: #3f3e39;
  text-shadow: 0px 0px 2px rgba(0, 0, 0, 0.9);
}

Changed to

/* TOP BAR */
#panel {
  background-color: green;
  /* transition from solid to transparent */
  transition-duration: 500ms;
  font-weight: bold;
  height: 1.86em; }

and

/* panel color */
#panel.solid {
  background-gradient-direction: vertical;
  background-gradient-start: blue;
  background-gradient-end: blue;
  text-shadow: 0px 0px 2px rgba(0, 0, 0, 0.9);
}

since i have changed the colors to green and blue in above. I should see top bar as Green color when no window is maximized. and Blue color when a window is maximized.

here are some screenshots.