XLS file is not exported correctly?

[B1tByByte]

We are exporting data into an XLS file to send to our partners for importing it into their system. This is how we are exporting the file:

Excel::download(new exportBillingReport($data), 'billing_report_' . date('m.d.Y.H.i.s') . '.xls')

Class is defined as:

class exportBillingReport implements FromArray, WithHeadings

The exported excel file (lets call this Version A) opens in MS Excel in 'compatibility mode', and when we simply hit the SAVE button (no changes made to content) the filesize increases substantially (let's call this Version B).

We send Version A to our partner for them to import, they have issues reading the file (see below for what they reported)
When we send Version B to our partner for them to import, it works just fine for them.

Issue they said they are facing: "It appears the file that failed is using a foreign character encoding. The fields must contain only standard ASCII characters. For example the word "pcn" should be 3 bytes. However, we are receiving 6 bytes of data."

Any ideas how we can fix this?