When I paste the following into a command block, activate it and open the book:
/give @p written_book 1 0 {pages:["[\"\",{\"text\":\"Heading\n\n\",\"bold\":\"true\",\"underlined\":\"true\"},{\"text\":\"Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor.\"}]"],title:"Bug Example",author:LiquidSonichu}
The first line of the paragraph is bold and underlined, even though no formatting is specified for it. It also spills off the edge of the page, unlike when I format it directly. The rest of the paragraph is plain, as expected.
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Comments 3

Confirmed for
1.8.4
And definitely not a duplicate of MC-55373
In MC-55373 it is said that
/give @p written_book 1 0 {pages:["[{text:Hello,underlined:false},{\"text\":\"Heading\n\n\",\"bold\":\"true\",\"underlined\":\"true\"},{\"text\":\"Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor.\"}]"],title:"Bug Example",author:LiquidSonichu}
Should work because the first entry with *underlined:false*
should now be the parent, however this is not the case
Same with
/give @p written_book 1 0 {pages:["[{text:Hello,underlined:false,extra:[{\"text\":\"Heading\n\n\",\"bold\":\"true\",\"underlined\":\"true\"},{\"text\":\"Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor.\"}]}]"],title:"Bug Example",author:LiquidSonichu}
It appears that rather the inheriting is broken or the book just displays it wrong

Duplicates MC-62866
Possible regression of MC-57843 or dupe of MC-55373