Bug when players start Adventure Mode

This is intentional, adventure mode doesn’t allow mining and placing blocks.
What you are thinking of is most likely Survival mode.

Hi!
Thank you for your report!
However, this issue is Working As Intended. 

Adventure Mode will restrict Players from directly breaking or placing most blocks.

Please note, that mechanics of the game may change between updates. Things such as graphics, sounds, world creation, biomes, Redstone, villagers, and animals may not work the same in current versions. 

Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki