Currently a checkbox setting type wouldn't consider multiple user selections. It works pretty much just like a radio button pretending to allow multiple selections when it doesn't.
Yep, noticed that in 1.6 - still not fixed and php optionscode has to be used.
Hi,
Thank you for your report. We have pushed this issue to our Github repository for further analysis where you can track our commits and progress with fixing this bug. Discussions regarding this bug may also take place there too.
Follow this link to visit the issue on Github:
https://github.com/mybb/mybb/issues/1947
Thanks for contributing to MyBB!
Regards,
The MyBB Group