I know this is stale, but I wanted to add something. I am by no means a graphic artist or even remotely knowledgable when it comes to all things Inkscape. However, I compared an SVG from Inkscape that displays correctly in powerpoint to one that does not, and the difference I found was that the disappearing paths were listed as a "use" rather than a "path" if that makes sense.
Apparently, these are clones (How to convert <use> objects to normal paths?). When I followed the instructions listed in that link, my SVGs started displaying their text correctly. I found it easiest to select one of the culprits, go to edit > select same > object type and then use the Alt+Shift+D shortcut to convert all clones to paths.
As it relates to this question, I can confirm that converting the SVG to a shape in Powerpoint Version 1908 Build 11929.20254 works for me. Also, I no longer lose the text after closing and reopening the file.