Thank you for reaching out. In general, if a job post disappears from your proposal tab, it means it got taken down for violating ToS.
However, I checked your account and can confirm the job post is still active and it shows in your "Submitted Proposals" section here.
I appreciate you reaching out to us for help.
To confirm if you've successfully sent a proposal, you may go to Find Work at the top menu, and then click Proposals. You should see it under the "Active" tab or "Archive" tab. Look for the job post and open it.
To assist you further I recommend taking the lessons below to learn more about Upwork and how it works.
I hope this is helpful to you. Thank you for posting in the Upwork community.