weird.
can't see how it's anything endemic to the ender 3 - so that's good, and also bloody unusual :-)

Cura will often do weird things, but if you are having the same issue with prusaslicer (basically slic3r) that's odd.
I mean the prusa slicer is optimised for prusa printers which are vastly superior to yours and have a whole bunch of features your printer (and most other printers) does not have.

But why it's reducing diameter at that point - I honestly can't imagine.

Does it show up in the preview at all ?

have you got the stl and I'll run it through simplify3d and see what happens.

haven't played wit prusa slicer yet. Be interestign to see what it does with my ctc bargain basemet i3 :-)