"lest the LORD see it, and it displease him, and he turn away his wrath from him" (24:18). Is the reason not to gloat so that your enemy still gets what he deserves?

The verse gives its reason and stops there. It follows straight on from "Don’t rejoice when your enemy falls. Don’t let your heart be glad when he is overthrown" (24:17), and says, "lest the LORD see it, and it displease him, and he turn away his wrath from him" (24:18). It doesn't say what follows once he turns away his wrath. Falls and overthrown are the two verbs of the verse before: "for a righteous man falls seven times and rises up again, but the wicked are overthrown by calamity" (24:16) (tap falls). The book speaks of this elsewhere too: "He who is glad at calamity shall not be unpunished" (Proverbs 17:5), and "If your enemy is hungry, give him food to eat" (Proverbs 25:21). The notes on Proverbs 1 set 24:17 beside Wisdom's laughing (Proverbs 1:26), and the notes on Leviticus 19 look at Jesus's words, "love your enemies" (Matthew 5:44). These notes don't say more than the verses do, and don't name anyone's enemy today.