My ex ended things but won't leave me alone?

He broke up with me because he 'couldn't see a future with me', and, 'things were different from when we first started dating.'
I'm not petty about the breakup at all, he has a right to end things afterall. But I was ovbiously shocked and upset, as it was quite out-of-the-blue.

Soon after the breakup, I muted his posts and all of his social media. We were still following eachother, but I couldn't see what he was up to, but he could still see my posts and storys.
I started doing things for myself, which I had wanted to do for a long time, but didn't as he thought it was a waste of money, etc, etc, and I didn't want to upset him.

I was posting these things to my IG story, and adding them to my highlights as I went along posting.

It had been 2 weeks since I had last posted anything on my story, and I got a notification saying that he had liked my story. Sent me down a line of panic for a miniute because I was anxious I had posted something by accident.

Turns out he had liked an old story via a highlight, he had liked a selfie of me with some freshly fitted hair extensions (which was something he was against me doing), with a song playing "Reminder" by the Weeknd.
The lyrics were "Every time you try to forget who I am, I'll be right there to remind you again" (or something, which I had used purposely ngl).

But I thought it was weird by out of everything he could've liked, it was this post. My face, with something he was against, espically with those lyrics. I don't think it was done by mistake, I think it was done purposely.

I didn't like the fact it panicked me, and afterwards I removed him off every social media. It was the best move for me as everytime I then posted, I wasn't overthinking whether he would like it or not.
It had been a few months since I had removed him, and I was doing really well. Then, randomly again, I saw a notification on insta saying he had started following me again.

He had started following me a day before his birthday, so I don't know if he thought I would wish him happy birthday? But he's not messaged me. And I'm defineltly not messaging him.

I don't understand why, despite me completely removing myself from his life, he's doing little things like this? And I don't understand what it means, if anything?
I wouldn't even think of liking his posts out the blue, or refollowing him if he had removed me on social media. I don't have a reason to, so thats why I'm wondering what he's trying to achieve?

I don't have any friends to talk about it with, other then my mum who just says to ignore it. Which I am doing, but I thought I would comment on here to see if anyone else can disect his weird behavior and tell me their opinion on it?

I know that I probably am overthinking it, but its just the fact that himself as a person, he's quite mature. He had good communication, thought of me, had a good work ethic, and overall was a genuine person.
So I don't think he's doing it because he's toxic, or controlling, or anything like that - simply because he wasn't like that when we was together... which is why it confuses me even more.

Does anyone have any opinions on why he's doing things like that?