Have a document open in Affinity Publisher (APub) - e.g. sample document provided.
Steps to reproduce:
Select an image on a page (e.g. photograph)
Right-click and select Layer Effects...
Select an affect from the left which offers Blend mode (e.g. Gradient overlay)
Select the Blend mode dropdown - dropdown opens
Reselect the Blend mode dropdown - dropdown closes, but is still selected
Use up/down cursor keys to change the selection.
When using the cursors to select a blend mode from the closed dropdown, the effect is demonstrated on screen on the selected image, but is not displayed/identified to the user. Only if you press ENTER will the selected effect be identified.
Use the open dropdown to make selections
Other notes and observations:
When using the expanded dropdown for Blend mode you can hover your mouse cursor over a displayed mode and APub will display a rendering of the highlighted blend mode. However it is not truly 'applied' until you click it. With the dropdown expanded, you can also use the keyboard to highlight a blend mode, but it is only applied if you press Enter, or click on the mode in question.