At no time does any client have "STEAM_ID_STOP_IGNORING_RETVALS" as his/her/its Steam ID.
GetClientAuthId() is the function that Sourcemod plugins use to retrieve a client's Steam ID. (GetClientAuthString is the older version of it)
The function features a return value, true if the operation was successful, and false if it was not. If it ever returns false, it usually means that the human client is not yet authenticated via Steam. If the function returns false, then the string buffer does NOT have the Steam ID put into it, because there isn't one (yet).
However, some newbie plugin developers weren't checking the return value. They just assumed that the function always worked, and passed the empty string to whatever other functions they used it with. Worse, in the days of decl, garbage data would be used as a "steam id," potentially causing server crashes, storing IDs that looked like gibberish, etc.
So, the Sourcemod developers decided to prevent that. If GetClientAuthId/String were to ever fail and need to return false, it no longer leaves the string variable alone. Instead, it loudly tells plugin developers to stop ignoring the return value, with the message "STEAM_ID_STOP_IGNORING_RETVALS"
If a plugin is telling you that a player's Steam ID is "STEAM_ID_STOP_IGNORING_RETVALS" then it's a really shitty plugin.
You DO seem to have another issue happening, but it's important to note why you're seeing "STEAM_ID_STOP_IGNORING"
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