Welcome to the forum!
I haven't heard of this problem before. And I don't use any kind of cutter. So all I can really do is use my own sense of troubleshooting, to try and narrow down the problem.
When you say that Signcut doesn't see it, do you mean that you can't see it in a preview? Sometimes in Inkscape very thin lines, which are perfectly horizontal or vertical, can't be seen. Then we just have to move a node by even just a fraction of a pixel, and the line appears. If it's a preview that you're seeing, maybe it has a similar problem?
Have you actually tried to cut something with a capital I? If not, it's probably worth a test, because maybe the preview can't display it for some reason, but it would still cut properly??
Can you show us a screenshot of whatever you're seeing?
What file format do you send to Signcut? Does it accept other formats which you could test (to try and find out if the problem might be with a file format).
Have you tried using another graphics program to send some text to Signcut (to find out if Inkscape might be the source of the problem, or if the problem is with Signcut).
And as far as the text, do I understand that all sans serif fonts have this problem with capital I ?
I guess at this point, I'm leaning towards this being a display problem with the preview. But let's start with these questions and go from there.