Create a command block with the following text:

git-Spigot-ac83d69-e13d119 (MC: 1.12-pre6)

give @p minecraft:sign 1 0 {BlockEntityTag:{Text1:"{\"text\":\"OH NO!\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"minecraft:tp @p 100 49 0\"},\"bold\":true,\"color\":\"dark_red\"}",Text2:"[\"\",{\"text\":\"The \"},{\"text\":\"flip\",\"obfuscated\":true,\"color\":\"aqua\"},{\"text\":\" broke.\"}]",Text3:"{\"text\":\"Right-click\"}",Text4:"{\"text\":\"this sign!\"}"},display:{Name:"End Spawn"}}

Add a button and get the sign.

In Vanilla, this works as expected and the second line has the "magic" text obfuscated as expectd.

Spigot 1.12Pre6, the sign appears as follows:

OH NO!
["",{"TEXT":"The
Right-click
this sign!

This bug may be related to https://bugs.mojang.com/browse/MC-41410 which appears to be related, but appears to work correctly in the current Pre6 vanilla.

Note, the example "/tellraw @p

" works as expected for both Vanilla and Spigot current pre-release 6.