Yeah, I'm making this a new post because searching through the manymany threads that already exist on it has been a huge waste of time.
Problem: Text frames in Affinity Designer inexplicably have a white background. No matter what the fill color or outline color of the text is, pasting, pasting without formatting, retyping text doesn't help. Every new text frame ALWAYS has a white background. There's no way to change it in the character studio and Designer doesn't have a text frame studio
open a new blank document,
go back to the original,
delete all of your text layers,
copy the remaining layers and art boards,
go to the new document.
paste them into the new document.
Re-create all of your text boxes.
Why it happens: for me it was an issue caused by opening a template my printer needs me to use. Apparently the document was setup to have white backgrounds in text boxes and since Designer shows the background frame color but has no way to get rid of it, all of my text boxes had white backgrounds.
Other solution if you only have a couple text boxes:
Create a new shape layer,
set the background to transparent,
convert it to a text frame
then paste the text
repeat for every single text box
I've found thread after thread about this issue here, on Reddit and elsewhere going back to 2019. Hey, Serif. It's been 3 years! just quit showing text frame background color at all if you aren't going to give us any option to control it.