With the following diff the functional test would pass. Fix by testing the default value.
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -1142,7 +1142,7 @@ static RPCHelpMan gettxout()
uint256 hash(ParseHashV(request.params[0], "txid"));
int n = request.params[1].get_int();
COutPoint out(hash, n);
- bool fMempool = true;
+ bool fMempool = false;
if (!request.params[2].isNull())
fMempool = request.params[2].get_bool();