The Bug:

Players without operator permissions cannot normally place blocks inside spawn protection, however, scaffolding can be extended into it.

Please note that this isn't a client-side issue.

Steps to Reproduce:

1. Start a server, join it, give operator permissions to yourself, and switch into creative mode.
2. Set the world spawn to your current location by using the command provided below.

   /setworldspawn ~ ~ ~

3. Locate the edge of spawn protection, obtain some scaffolding and place it adjacent to, but not inside of spawn protection.
4. Switch into survival mode, remove your operator permissions, and grant anyone else with them in order to activate spawn protection.
5. Attempt to extend scaffolding into spawn protection.
6. Take note as to whether or not you can extend scaffolding into spawn protection.

Observed Behavior:

Scaffolding can be extended into spawn protection.

Expected Behavior:

Scaffolding would not be able to be extended into spawn protection.