Using thumb buttons as hotbar keys does not work as it should. When using them in the inventory they do put items in the desired hotbar slot when clicked over an item.
I can provide a video demonstration later if you want, right now I'm away from my gaming pc.
Thank you for your report!
However, this issue is Incomplete.
Your report does not contain enough information. As such, we're unable to understand or reproduce the problem.
Please review the guidelines linked below before making further reports.
You can also check out this video guide by @unknown about how to create good bug reports.
In case of a game crash, be sure to attach the crashlog from
[minecraft/crash-reports/crash-<DATE>-client.txt|https://minecrafthopper.net/help/guides/finding-minecraft-data-folder].If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki