What does gray line mean on google maps time line

When I look at my time line it shows the route I took but in middle of the route it goes gray? Then starts up blue again. On my mac I can click on line and it will show exact route. I can then click on hybrid space ( not sure of the word) I will take me through a video of route but when it come to where the gray line was it shows black until it gets to blue line again. Does this ahve something to do with location turning on and off?

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