Hi @Simon Probert ,
Welcome to Q&A forum!
According to your description, I noticed that you are using the currently latest version of Office 2016 for Mac. Due to the limitations of the test environment, I have done a lot of research and testing in the following versions of Excel. I found that this is by design.
When I set both the background color and the pattern color to two "lighter" colors, I will reproduce your issue.
It is recommended that you set the background and pattern colors to a "darker" color and try other Pattern Style to see if the stripes can be displayed normally.
Beisdes, if needed, I would suggest you refer to this page to submit suggestions on this issue to Microsoft directly.
Hope the information could help you. Any updates, pelase let me know.
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