Player seeing the entire map

Several times now one player has been able to see the entire map as if there are no dynamic lighting walls. This has happened in 2 different campaigns. Reloading sometimes fixes it but only temporarily. Any ideas why this is happening?

Hi Dungeon Master, Are you using Explorer Mode?

Yes

What browser is your player using?

Chrome on windows 10 64bit os

When a user's computer fails to render the Explorer Mode layer they will see the entire map.  The only solution I have found to this is to not use Explorer Mode. But, you can try using Permanent Darkness to see if that stays when Explorer Mode fails. I don't remember if it also failed or not.  To mimic the effects of Explorer Mode I suggest using light crumbs the players can see, and then put anything they shouldn't be able to see on the GM layer. 

Ok thanks for the info