There are schools of thought that recommend eating fruit on its own, saying it doesn’t digest as well when combined with other foods. I learned this in Ayurvedic medicine, where you should avoid it with dairy, for example. Eating fruit with vegetables is never a cause for concern, and I haven’t talked with many folks who say that combining fruit with oatmeal causes any issues.
In practice, if one is experiencing chronic digestive discomfort (bloating, gas, indigestion after meals), avoiding fruit with dairy can be helpful for some people. It’s a tool to keep in the back of your head, especially if one is not absorbing their food that well. I used to experience a lot of digestive bloating if I ate an apple or berries after meals, but that was back in a time when I had chronic digestive bloating every day for other reasons.
For you, if you have been doing it for years, and your digestion feels great, I wouldn’t worry about it too much.