Hello, BlueSky77#
Welcome to Microsoft Community.
I've tried in Excel as you described and I encountered exactly the same problem. So it is probably a bug which needs to be fixed. I'm going to submit this problem to Microsoft Developers Team. And you can also report this to Microsoft. We’d love to hear from you about what’s working well, what’s not, and any ideas for improvements including possible new features and functions. Your input will help us improve the user experience for you and all Office users worldwide.
The quickest route to get your comments to our developers’ ears is right from within any Office app. They read customer feedback daily.
From your Office app, go to Help > Feedback.

or
From your Office app, go to File> Feedback.
There is a temporary workaround before this issue is fixed: In Text Options > Text Fill, change Transparency to 100% so that the texts are not apparently visible.
I understand this may not be the outcome you are expecting, but I hope I've explained this issue clearly.
Best regards,
Yuhao Li| Microsoft Community Support Specialist