How can I get useful feedback when collaborating on research proposals?
Answer 1:

Ask for explicit feedback (maybe after the deadline is passed and crunch time is over). Giving feedback on your work is part of the implicit deal that a senior person enters when enlisting the help of a PhD student on things like that.

Answer 2:

In addition, re-engineer what they did to your text. You sent a full page of content and little of it made the cut. That is valuable information: apparently your text was, for one reason or another, not suitable. Read very carefully what the seniors decided to write instead: did they focus on different aspects? is their writing more polished in terms of language? is the tone different? was your original draft not aligned well enough with the rest of the proposal? is there anything else that reads notably different?

What I did when I was learning the ropes (actually, I still do this sometimes when I work with somebody better than me at grant writing) is to very carefully re-construct how they build their argument, what they write, and what they don't write. If in doubt, I ask them directly, but I feel the exercise of figuring this out myself is a better learning experience than simply asking for feedback.

Another thing to keep in mind of this happens regularly is that maybe you need to ask more, or better, questions before starting to write the text. If the PI asks you to write about related work, discuss in advance what fields you should cover and in what detail. If you should describe a specific work package, run your thoughts on what it should entail briefly by the PI before writing it all down. A quick meeting is usually more productive than multiple iterations of text writing, and being efficient is usually very important when writing a grant.

Finally, figure out whether anything related to your text actually made the cut. Sometimes, one just finds out when putting it all together that a specific section that sounded like a good idea during planning simply does not fit the proposal after all (for content or space restriction reasons). This is unfortunate, but still happens sometimes. In that case, maybe your writing was perfect, but your section simply still did not make the cut. In this case, move on.