Unfortunately, it's not a bug, it's a feature.
As you noticed, the 'hide' and 'unhide' function of the + and - grouping
buttons won't work if the sheet is protected. That's the way the protection
works.

"Gabriel De la Garza" wrote:

I have an excel file that is shared with multiple users. Some of its contents
needs to be protected, so I went to Tools, Protection and I protected the
worksheet, leaving unlocked several rows and columns that needs to be updated
by users, but still some other rows and columns remain protected.

As the file is to long (about 1000 lines) and very wide (it reaches column
IQ), I use the functions to group and ungroup several rows or columns to
quick expand or shrink the file (hide or unhide rows and columns). The
problem comes while the file is shared and protected, the group and ungroup
format functions are not allowed to use.

The cells that can be modified, were previously unlocked (select the cell,
click format cell, protection tab and remove the checkmark at locked, the
placed the protection pworksheet password).

Thanks for the help.