I just did a little testing of this action, and it is not sending the extra line in the text parameters used. It appears to be something being added by the advanced-uri plugin at Obsidian’s end…so you would need to contact the creator of that plugin to see if there’s a way to avoid that happening. I didn’t see anything obvious in their docs.
There are other ways to send data to Obsidian, however. You might want to look at some of the other options in the Obsidian integration guide. If you are writing directly to files in the vault, rather than using URLs, that would not be a problem.
That action has a single “File” action step. That step has a “Write Type” option, which is set to “create”. If you edit the action (tap and hold on it in list > Edit on iOS), tap on the steps, then the “File” step, change it’s write type to “append”.
Append actions will create the file if it doesn’t exist, or add to it if it does.