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There could be a couple of different reasons for that, and I'm not sure which one might be happening. First, select the text, and look at the status bar (bottom of window in the middle). If it says "Group of.....", try opening Object menu > Fill and Stroke, and click this button: :eofr: . If it's fixed, good news!
If not fixed, then you might need to do Object menu > Ungroup, before you can try to fix. And you might need to do that a few times. Instead of using the menu, there's a button on the command bar, that looks like this:
Click as many times as you need, until the status bar says "No more groups to ungroup". If you click that a few times, but the status bar is still reporting some groups, you might need to use a special extension which will ungroup them all at once. Unless you have the current development version, you'll probably need to install the extension. But let's hope it goes quickly, or that you don't need to ungroup at all.
After everything is ungrouped, or if it doesn't report a group in the first place, try selecting all the text and do Path menu > Combine. If it still isn't fixed, you might need to do some node editing.
I'm curious why you didn't create the text originally in Inkscape? Typically if one needs a DXF file, they don't need any fill, because they're going to use the paths for cutting.
So if you don't need them for cutting/plotting, it might be easier to create the text fresh in Inkscape, than to node edit the text - especially if it's a large amount of text.
Anyway, let us know what happens