This issue is related to MC-87143
So according to the "fixed" issue named above it seems to be intended that you have to write JSON-text on signs now.
So that command works just fine:

/setblock ~ ~1 ~ minecraft:standing_sign 0 replace {Text1:"[\"test\"]",Text2:"[\"\"]",Text3:"[\"\"]",Text4:"[\"\"]"}

while the following just produces an empty sign:

/setblock ~ ~1 ~ minecraft:standing_sign 0 replace {Text1:"test",Text2:"",Text3:"",Text4:""}

However, what is probably not intended is that the 2nd command produces the following error text in the command block output line even though the sign gets placed:

An unknown error occurred while attempting to perform this command