I ran into this as well. And I'm surprised I didn't report it.
self.verify = verify if self.builtin.convert_to_boolean(verify) != True else None
What happens here is that if verify is considered False after converting to boolean, then self.verify is set to the original value, the string "false".
If you do not specify verify on each request, then RequestsLibrary is sending verify="false", which requests is not expecting.
As a workaround, use a real bool
Create Session alias=test url=${SERVER} verify=${False}
Please do also keep in mind that this verify setting is RequestsLibrary-level setting where it should be a per-session setting.
What this means in practice is that whatever the value is for verify on the last session created will be used as the default value for requests regardless of session.
So if you create a session A with verify=True, then session B with verify=False, then when you send requests on session A without sending the verify parameter, then verification will not happen.