Additional spaces in .ind and .idx files

I am really noob with LaTeX, and I am running into an issue with an index:

In the .ind / .idx files, spaces are added to some entries, for a reason I cannnot figure. For instance, while the .tex file has

index[IndexInterp]{textbf{Bethsames }! domus ueri solis}

in the .ind file this entry becomes:

item textbf  {Bethsames }
subitem  domus ueri solis, 279

As a result, this entry does not appear in the right place according to the alphabetical order.

I would be grateful if you had a way to fix this.
I work with TeXWorks on Windows 10, and typeset with XelateX