What did you do?
Open the Factoriopedia, and go to the beacon page.
What happened?
The horizontal axis of the plot of the beacon transmission strength has 2 times the 'tick' 10:
This is also why the 'hover-over-lines' don't align properly (the tooltip says 9, but the value seems 9.4):
What did you expect to happen instead? It might be obvious to you, but do it anyway!
Should just be …, 8, 9, 10, 11, 12, … And the lines should align with the integers on the horizontal axis.
Does it happen always, once, or sometimes?
Always happens.