The response looks correct to me? Also I think it works in VSCode (the bug did not have the original source file to repro with so I was guessing from the returned edits)
Why do you think there is an overlap? It has two insertions at the end, but they are not overlapping, they are just insertions that should happen one after the other at the same location.
The spec is very explicit that this is allowed

All text edits ranges refer to positions in the original document. Text edits ranges must never overlap, that means no part of the original document must be manipulated by more than one edit. However, it is possible that multiple edits have the same start position: multiple inserts, or any number of inserts followed by a single remove or replace edit. If multiple inserts have the same position, the order in the array defines the order in which the inserted strings appear in the resulting text

Also note that a file without a \n a the end is not correctly formatted according to gofmt, so the formatting call will attempt to fix it for you.
cc @stamblerre

I was believing there are overlaps because of the two edits targeting the same range. But that might fall into the However clause you pointed out.

Slightly off topic, VSCode itself works usually doesn't justify there isn't any issue. There are far too many implementations that are using VSCode as standard rather than the protocol itself, which makes plugins authors life harder.

Thanks for looking into this!

Ok. Glad I could help.
I was not trying to imply that VSCode is always correct, just letting you know that I had attempted to reproduce it on the editor I normally use and was not just replying without checking, because it frustrates me when people do that to me.