Does it work if you use a different method for the posterior: e.g., p_direction(posterior, method = "kernel", null = 0.5)?
-----EDIT-----
Just realized that your question and issue is with the plot function, not the actual p_direction() function… that seems like an issue with the see package then, and I don’t see anywhere in that language for the null value (see/plot.p_direction.R at 6434c554799bc4cdc98874559fec970faab42bb0 · easystats/see · GitHub). You may need to create the desired plot in ggplot2 instead