It's currently impossible to write on signs on Minecraft Xbox One. I can't tell if the issue is with the console or with Minecraft.
The issue seems to be that when the Xbox keyboard pops up after putting a sign down it gets rid of the sign interface meaning you can't write signs on any other line besides the top one. (Pressing START or B backs you out of the sign and the keypad as shown in the video.
What should happen is the sign shouldn't disappear when the writing has been confirmed.
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We're tracking this issue in MCPE-156575, so this ticket is being resolved and linked as a duplicate.
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