For a long time now, the sign hitbox has been different from the text that fits on the sign.
Obviously the sign's hitbox is one block. However, the model is bigger than one block and thus gets overlapped when a block is placed on top of it.
This causes the first line of the sign to be unreadable. You might not consider this a bug, but it is definitely not a feature either.
Examples given in the attachments.
Linked issues
duplicates
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Comments
CubeTheThird
Duplicate of MC-3904 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.
This is a mild inconvenience. Just for compensations sake; confirmed.
Hopefully something might be done about this, but I don't think many people are placing signs with text on them the way you are so I don't think it would affect many people. Besides, BdoubleO100 would be sad if his cool floor got ruined.