PDF imports can be extremely complex in Inkscape. There can be numerous multilevel groupings and clipping paths. You should check the content of the Objects panel. It shows groupings and clippings (if you learn from the documentation how they are shown).
Only guessing is possible without knowing the structure. PDFs can be built in so many different ways. To have a possibility to get a pinpoint answer instead of guesses show the Objects panel here so that groupings are expanded to visible.
Guess 1: Select all and apply Extension > Arrange > Deep Ungroup to get rid of multilevel groups. Then select all and apply Object > Clip > Release few times. Something can become visible.
Guess 2: You can also try to delete that text box and write a new which isn't behind anything nor clipped. Clipping paths can affect to a whole group or layer.
If you succeeded to select the right object try to cut it into the clipboard and paste it onto the artboard. Select the layer in the objects panel before pasting to get the pasted object on top.
The pasted object inherits the original, possibly group level clipping path. Apply Object > Clip > Release to make your object free.