I think so.
I did a few tests by turning off my device and I no longer had access to the offline mode that I had downloaded before.
Unless a Windy developer explains otherwise, this is a limitation of using the Offline-mode.
Before this feature was available I used to make a screen video of the Wind layer when pressing the play button to get the time scrolling. I don’t know if the screen video is available in your device, but it is a standard feature on iPad.
It is a way to keep the forecast available on your device even if you turn it off. You just need to play the video.

Which GRIB did you download? There is no need to obtain third party GRIB files to use this mode.
Or do you mean you downloaded the data offline?

Thanks for your answer.

I downloaded the data offline.

So according to your answer, if I downloaded 4 days of offline forecast and during the trip my laptop accidentally turns off, do I lose the entire forecast?

Which GRIB did you download? There is no need to obtain third party GRIB files to use this mode.
Or do you mean you downloaded the data offline?

Offline mode is explained here:
https://community.windy.com/topic/26510/help-us-to-test-windy-com-in-offline-mode?_=1711452965988

Access to Offline mode is in the Menu

Unfortunately, if you turn off your device, offline mode is deleted. It is therefore necessary to download the data offline when you are on board and keep your device on.
But it would be helpful, if possible, to keep the downloaded data on the device when it is turned off.