Hi @s44 ,
Do you want to filter records by Person column?
It seems that you cannot use SortByColumns() in the "Person or Group" column of a SharePoint list.
In addition, Is it necessary for you to use SortByColumns()?
I have made a test for your reference.
1.Here is my SharePoint list.

2.Select the Gallery Control and apply the following formula on its property as:
SortByColumns(Filter(AddColumns(recruitrequest,"applicantdisplayname",applicant.DisplayName),StartsWith(applicantdisplayname,TextSearchBox1.Text)),"applicantdisplayname",If(SortDescending1, Descending, Ascending))

3.In addition, you can try to use Sort() function.
Sort(Filter(recruitrequest,StartsWith(applicant.DisplayName,TextSearchBox1.Text)),"applicant",If(SortDescending1,Descending,Ascending))
Result Screenshots:

Best Regards,
Charlie Choi