Can't set up single column of icons with a Fence

I've been using Fences 2 with Windows 7 then Windows 10 for some time (both trouble-free), and have always been able to set up icons so that they appear as a single column with a Fence.

Fences has suddenly started putting all the icons into 2x columns, even if the Fence is only wide enough to display 1x column. Manually dragging the icons back into a single column doesn't work ... the software just reshuffles all the icons in that Fence back into 2 columns.

The problem persists whether I have 'Keep Fences sized to even multiples of icons' checked or unchecked.

I've also just tried upgrading to Fences 3.0, but that hasn't solved the issue  either.

Any ideas?