Not sure what you're doing wrong, but the Synth.setFixNoteOnAfterNoteOff() is precisely for this use case and should work like in this example that delays note ons by a second and prevents hanging notes if you press the key shorter than a second.
HiseSnippet 910.3oc2V0saaaCElx1pX1cqncnnnWJLrKbAJJraS6FvtnIwwtvX0IFQo+bWAC0QwDghTihxsFE6cbOJ8MX6PJ6JoFiTGitUr5KLz4OxOdNemC4TshAYYJMwq8IKRAh226GtPZlMXFkKIiOf3cC+IzLCnCJTs+hTZVFDQ77Z9LqBu1sHtee3o6SETICJUQHuTwYvy4IbSo1o696bgXDMBNgmTw6c1cLSIGnDpbDOM86QRoryomAGRst0vm3csgQbiRGZnFHC8YeUzhvYp2JK7+k7L9oBvJzmDhKTgZxfYbQzzUm0LBwyeZ4IuYwI+19S3Q7OpuLCbSmgfxHplC7ZbYPp+U.RdUfTqBHcK+PllmZJsXwy08GKwBRLES0UgRgujF+jm+.E5gz7fD54vHMJ7wH59jd8te.9289sNtJ8ChfXPOfJDmhY6rtwTQFfF6rzbFXFwe2gJCbjbuXbUbeFG20nys9EmKYFtRFnjEN08dcdem1SP7hkNb0EzECmifo6N6z2sss67mcB9zvvEzE2mXydPzJg.zq0rk.our.6JySNEz2OXNUfvckiXFtdY6ZaVYiUjWq3nRNVxMGkBKkGoDQ1xg86KVjIKKL3WuX7ATC0V2WpC8KEzFtENdG.ywFmBVPa+CfryMpTr04BTDjbphxETScFqs0boALeTilXoBxLtYQ0V2q.Mt2kRi2THdK+obCa15wXi0fQLS8uAFW17+C9CiiAloDfs7G85ssSey29aTr8W2OjKAWCmayuiSN3Uz4Pvy.InsIu9Wxv2+ZSG9ltwCeOhYvs+DMUlkpxpsvgPB+DkzlEJU9hLbTC7GGaqlU0OPQ0q0zGd5H7Pt1XBolbsiVrWhJWZpQCZVuNzZyZcq2QTw+0LJt0WgaG9xzV8Uryu1EXsWgwPdRp.FJmCBb7lCi+HNOKllKLqzVmXOQIUoyTRNqZU+Xvn4mcFnqh80df1yXvKxJ0b6cOFD.sJC9m284Hyipw7Drk4hq9k4qsdcW+B3FX6FC9+6D6leaOw9XUtgKOaBEIguCe83g4Ig3fRFf6tTBBbw88ZXG1UH2yJWLoTF4D9a72Ri8sxdKM1ekw+S1iDJSqdCq3sQVd524zfmao6M1swG6ixA8It2KUsVlfS4dCiUeotPfObaC7Qaaf6rsA93sMvmrsA9Kaaf+5mOP6kP6kaTIEslDxjoCcOvzyanjhrbWGA4e.vLdxN.