The filtering on the spreadsheet component is great!!
There is however one behaviour that causes confusion for end users, and that is filtering when merged cells are present in the selection.
An example is available in the following dojo:
Open the dojo and filter column A on a1 and you will only see only b1 in column B, but the b1a will not be shown.
To be clear - This is also how Excel behaves... (which is of course your prime aim so its not a bug as such)...
Interestingly google sheets stops you putting filter on merged cells / stops you merging on filtered column.
The Excel behavior is discussed in the following threads:
Is it possible to implement something, so the end user experience would be improved? Maybe when merged cells are present :
a) if they click filter, this is detect and user is warned of this behaviour
b) if they click filter, the sheet is changed into an unmerged version (which repeats data in merged cells) as in the excel examples above.