I'm not permitted to show screenshots of the application that I'm working on so I'll use the sample application that Star-Dust provided at
https://www.b4x.com/android/forum/threads/b4x-xui-sd-flexgrid-table.100897/.
B4A Sample.zip
I'm running B4A version 11.80 on Windows 10 Professional 22H2.
This is a screen shot of it running in the emulator. It appears exactly the same on my phone.
As you can see there are no grid lines. The only effect that the "Border Cell Color" seems to have is to set the color of the button border in the first column.
I have not altered his source design or code. Here are the custom properties:
Presumably this does produce grid lines for others.
Any idea why the grid lines aren't appearing when compiled on my system?