Well, I got a Belgian history course at university where there was a chapter on the colony. I don't remember whether or not I learned it in secondary school as well. It could be that we didn't, but we skipped over more important stuff as well. It could be argued that Leopold II's Congo is a notable part of Belgian history, but it could also be argued that it actually wasn't all that relevant (because these things happened on a different continent, also because they had little influence on anything else that we'd learn about, and also because the colony was privately owned by the king and had no ties to Belgium).
We did extensively analyse the song We Didn't Start The Fire by Billy Joel in the 6th year, in which the murder of Patrice Lumumba was mentioned.
I understand the logic that it doesn't really feel like Belgium's history because as you said it happened on a different continent and it's now a different country. However, I think with that logic you could freely omit most foreign intervention from the history books.
There was a ton of wealth generated in Belgium at the cost of the Congolese. This seems like a significant omission, especially for a country with only a couple hundred years of history. At least I understand if the UK or France were to examine colonization in school it would take forever because they practically colonized the whole world. Still, it does surprise me to hear some people from the colonial powers not to tackle colonization at all. Many problems in the world today are rooted in their colonial history.
Once again the US is not innocent in this. While we have become better at recognizing the genocide of Native Americans when colonization first took place on the continent. We still ignore the terrible acts the US engaged in as we moved westward and claimed more land.
My Real Redneck friends