Hi this may be similar to #89
Every powerpoint I make needs to be repaired when opened by powerpoint.
It isn't just the ones that i have made for my own work it is also the sample folder versions.

I used composer to get "phpoffice/phppresentation": "dev-master"
I have run the simple sample 1 file. (this is the test-case but all files i create require repairing)

I have tried OpenXML SDK2.5 productivity tool by microsoft and it states there is no problem with the pptx.
I have tried opening with: powerpoint mac v 15.26, powerpoint mac 14.6.7, windows 10 office 2013 powerpoint, keynote refuses to open the file, libreoffice opens it without complaints.
When the files are opened there are no visible issues.

for reference i have enc. a link to the file i have created. i hadn't changed the default sample files.
except to make a 'results' folder with write permissions.

Its probably a setup thing, but any help/suggestions would be appreciated.

Sample_01_Simple.pptx

I went ahead and wiped the installation and re-installed it. i had to change the path of require_once DIR . '/../vendor/autoload.php'; on Sample_Header.php to get the samples to work. But still had the same issue.
The ODP format doesn't have this issue.