That was probably done by fiber[1] the code specifically took the param from it in the function passed to the Get(path string, handlers ...Handler) Router function. c is the *fiber.Ctx passed by fiber to the handler. My code took the string from c.Param("name") passed it to url.QueryUnescape then another function which had a mutex around setting the key/value in the map. I got the hint it was slices and something modifying the keys when I found truncated keys in the key list.
My guess is fiber used the same string for the param to avoid allocations. The fix for it is just to create a copy of the string with strings.Clone() to ensure it does not get mutated when it is used as a key. I understand it was an issue with my code, it just wasn't something I expected to be the case so it took several hours and using the debugger to find the root cause. Probably didn't help that a lot of the code was generated by Grok-4-Code/Sonic as a vibe coding test when I decided to go back a few months later and try and fix some of the issues I had myself.