Seems to me a bug in PlutoUI, I can reproduce this - initially the value is 0.0, but when the slider is moved away from zero and back down it switches to Int64. A simple workaround is to actually fix the type of the Array:
xs = Float64[x0, ]
EDIT: looks like this is a known issue, with the problem being on the JS side, see here: Instability in return type of PlutoUI.Slider · Issue #64 · JuliaPluto/PlutoUI.jl · GitHub