From snapshot 18w46a onwards, if you're not targeting scaffolding directly, you won't be able to place it in a way that allows it to fall without a supporting block

Placing scaffolding without a supporting block beneath it will result in it falling like other gravity-affected blocks such as gravel or sand. This behavior applies both when players manually place scaffolding and when using the /setblock command. In snapshot 18w45a, it was possible to place scaffolding midair and have it fall immediately, which was convenient for building scaffolding towers. However, starting from snapshot 18w46a, this capability is limited. Now, floating scaffolding will only be placable if you are aiming at scaffolding. Therefore, if players wish to manually place scaffolding that instantly falls, they need to create a bridge away from existing scaffolding until the distance block state is sufficiently high.

Reproduction steps

1. Erect a stone pillar.

2. Attempt to position scaffolding adjacent to the pillar to prompt its immediate fall

Observation: The reproducer noticed that it's impossible to place the scaffolding.

Expectation: The reproducer anticipated that the scaffolding would be placed smoothly as it was in snapshot 18w45a. It's likely that this functionality still operates correctly in Bedrock Edition.