Affinity Designer v2.3.1 on Windows
This error occurs in NEW and EXISTING files.
Create a line (curved or straight, it does not matter) using the Pen Tool.
Add decorations to end(s) of line.
Expand Stroke.
This will only work if (Stroke) ALIGNMENT is set to "Align Stroke to Center." If set to "Align Stroke to Inside" or "Align Stroke to Outside," the decorations will disappear.
Note: this same error occurred on the stroke when I created a box using the Rectangle Tool.
See below. I created 4 objects. Duplicated them below. Expanded the stroke of the duplicate items. Selected All to show the different in the diagram.
Top Row: Pen Tool (Align Stroke to Center), Pen Tool (Align Stroke to Inside), Pen Tool (Align Stroke to Outside), Rectangle Tool (Align Stroke to Inside)
Bottom Row: Decorations Preserved, Decorations Lost, Decorations Lost, Decorations Lost