What I’d recommend for determining where SVG problems stem from are as follows:
View the SVG in multiple web browsers. If the problem shows in the web browser, you may have an OmniGraffle bug to report to us. SVG files are readable text which means when there are issues, sometimes you can tell what the issue is exactly. The way I do this when I’m troubleshooting is to import the SVG into a new OmniGraffle document. Then I can tell what the problem object name is by expanding the layer sidebar and seeing how the SVG objects are named. I open the SVG in my favorite editor (BBEdit or VSCode for me), and then I search for that object. Sometimes the issue is the SVG uses animation, which OmniGraffle does not support, but web browsers do support. Sometimes the issue is with OmniGraffle.
If it shows correctly in a web browser, but not when you import it, you have a PowerPoint problem. I’d try exporting a different image format from OmniGraffle in that case.
Text not showing that shows in a web browser is an import issue. You can try to work around this by making many separate small text labels instead of relying on precise positioning, or you can report the bug in PowerPoint to Microsoft.
I hope this helps! Please let us know if you are seeing cases where what you export from OmniGraffle doesn’t appear as you expect in Safari and Chrome. Those are the cases we would want to know about and would have a good chance of resolving for you. Even if we can’t fix the issue, we may be able to help with workarounds if you’d like to send us examples of issues with SVG export, choose Contact Omni from the Help menu and we will be happy to take a look.