editing XLSX files - disappearing borders

A new problem has appeared - when editing a spreadsheet with borders, they suddenly disappear.

Steps to reproduce:

1) upload/edit a blank XLSX file ( https://sheet.zoho.eu/sheet/remotedoc.im, [ 'content' => curl_file_create($pathToFile) ])

2) add any borders anywhere in the spreadsheet, save it

3) upload/edit the same file again

4) do something with the contents, save it

Resulting file sent to saveurl will now have no borders, both in desktop office applications and when opening again in Zoho Docs.

I additionally tested a scenario where you edit existing borders and add new ones, and it looks like it just removes borders added 2 edits before, which is really weird.

For example, if I upload a blank file and add a left border to a cell, save/close/reopen and add a right border to the same cell, save/close/reopen the third time, the left border added in the first edit has disappeared, but the right is still there.