A PowerShell Profile Can Put the Proxy Back
Compare Windows PowerShell 5.1 with -NoProfile and inspect all profile paths when a removed proxy reappears in new sessions.
If a proxy returns whenever a Windows PowerShell 5.1 session opens, inspect the code run at startup. A profile can assign a default proxy, set environment variables or define request defaults again.
Compare the same request and account in a fresh session started with:
powershell.exe -NoProfile
This skips profile scripts. It still inherits its parent environment and remains subject to applicable Windows networking settings, so a remaining proxy does not rule those out.
In the ordinary host where the problem occurs, list the profile paths:
$PROFILE | Select-Object *
Check the existing files at those paths and any scripts they load. PowerShell has all-user and current-user profiles for all hosts and for the current host; later profiles can override earlier ones. Another terminal host may therefore run a different startup script.
Change the specific assignment that reintroduces the route, then compare a new session. Deleting unrelated registry settings does not remove a proxy that startup code immediately assigns again.