Why do I catch him looking at me, but avoiding me at the same time?

I’m friends with this guy and we used to be really close and hang out all the time but feelings got involved and it was easier/ morally right for us to keep distance. He’s in a serious relationship - that’s why I mentioned the morally right thing haha.

Anyways about 3 months ago after we grew closer again, he became distant and I saw him a lot less. We both have got new phones, for unrelated reasons, and haven’t given each other our new numbers or anything. I’m so sad about it but I know it’s for the right reasons. However, when he walks past my workplace (he works just down the street) I sometimes catch him looking at me through the window as he walks past. Sometimes this can be a few times a day. Whenever I catch him, he either half smiles, looks sad or looks away really quickly. His dad even came into my work the other day and told me how much his son loves me (obviously he doesn’t know about the feelings in the past, so I'm sure he meant in a friendly way) and that they think I’m a lovely girl. It was such a bittersweet moment - made me tear up a bit after he left haha 😩 Yesterday, my work colleagues saw him looking at me twice and I saw him looking at me another two times. One of which, he was actually looking back at me as he was walking away and smiled when I caught him and then looked down... Then today came, I saw him walk past a few times and he looked down everytime and avoided looking in the window.

Can someone please give me advice on what’s going on here? I’m worried I’ve done something wrong because he is avoiding actually speaking to me now. Before, we just tried to sweep our feelings under the carpet and remain friendly.
Why do I catch him looking at me, but avoiding me at the same time?
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