When I select a rectangular region, the selection highlighting only applies to the final line, rather than the entire rectangle. The cursor correctly stretches to the height of the rectangular region.
This happens on SciTE 5.3.7 using Scintilla 5.3.6 and Lexilla 5.2.6, installed via the standard Arch Linux package. It worked correctly on 4.2.1 installed on Linux Mint 19.
Selection visual choices were changed between 4.x and 5.x so, if you have customized settings, then they may produce different results. Remove them and customize again.
Rectangular selections are multiple selections with only one line as the main selection with other lines being 'additional' selections. The default settings use translucent black with the additional selections being around twice as transparent as the main selection so less visible.
These are the default settings to start customizing from.
selection.back=#0000001E
selection.additional.back=#00000010
selection.secondary.back=#00800010
selection.layer=2
Ah-ha, sorry I didn't think to reset the user settings first. I use a black background, so the black shading was invisible. I have it working now, thanks!