Size error in chart with single member group

When I have a group containing a single member markers are drawn oversize/not to scale with other groups.

dt <- data.table(x=1:10,y=rnorm(10),s=rnorm(10),cat=c('a'))
dt[1,cat := 'b']
plot_ly(dt, x=~x, y=~y, size=~s, color=~cat) %>% add_markers()

dt[2, cat:= 'b']
plot_ly(dt, x=~x, y=~y, size=~s, color=~cat) %>% add_markers()

Not sure if this is intentional or Iā€™m using it wrong.