Avoid area keeps disappearing...

[Daytona]

I've set an avoid area on my iCN510 to prevent routing down a short length of rather narrow road near a place we quite regularly go to outside Chester. The alternative route isn't much longer but the road is better for oncoming traffic.
Sometimes, the avoid area has vanished from the map (as it had yesterday) and I have to recreate the avoid area. How can this be? I've not changed anything on the unit since the previous journey (when it was fine). Neither had I the previous couple of times this has happened. No updates, no changes in Favourites, no change of maps installed, nothing at all.
Baffled....

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