Pinning and unpinning of topics is at the user level. This means the user is free to make changes to pin/unpin as they please.
There is however also a setting ‘enabled by default’ to unpin any topic once you reach the bottom. This may cause some confusion and may result in topics, we ‘leaders’ have set to be pinned, to get unpinned again. There is no way for us to make a distinction, each pin is treated equally, unless this behaviour is disabled forum wide.
Thankfully you can change this unpinning behaviour yourself through your ‘Profile Settings’.
Visit your profile page through your user icon at the top right
Now select the ‘Interface’ settings and look for the option to:
‘Automatically unpin topics when I reach the bottom’ and remove the check mark if you prefer to keep pinned topics pinned.
I am sure some will find this useful, while still allowing you to pin/unpin any topic as you seem fit.