Does the fast of one who wakes from sleep and finds a trace, of which he does not know what it is, after he saw himself engaging in sexual intercourse in a dream, become invalidated?

If you experience a wet dream (احتلام) but do not see semen (منيًّا), then ghusl (ritual bath) is not required of you. If you doubt the emission of semen, the default is that it has not been emitted, so ghusl is not obligatory until you are certain of its emission. Your fasting is valid in all cases and is unrelated to your wet dream, whether you saw semen or not.