The bug

Written Books that use Json scores do not update if the book was created after points have been added to a players score.

How to reproduce

Setup dummy scoreboard

/scoreboard objectives add bug dummy
/scoreboard objectives setdisplay sidebar bug

Create Book (via command block). The book that will work.

give @p written_book 1 0 {title:"Bug",author:"Steve",pages:[
	"{text:'Bug score: ',extra:[
		{color:red,score:{name:@p,objective:bug}}
	]}"
]}

Read open/read the book you just created.

Give yourself a point to be added to the scoreboard.

/scoreboard players add @p bug 1

Create second book (via command block(same command as before)). This book wont work.

give @p written_book 1 0 {title:"After",author:"Steve",pages:[
	"{text:'Bug score: ',extra:[
		{color:red,score:{name:@p,objective:bug}}
	]}"
]}

Read this book.

Both books will show "Bug score: 1"

Add more points.

/scoreboard players add @p bug 5

The first book will now show 6 points.
The second will only show 1 point.

If you reset your score the first book will go bank and the second will stay at 1 point.