Betta Fish: Any idea what’s going on with his side?

First photo is the day I got him, second is a month or so later, third is today (about two months with him now). Photo 4 is within the last couple weeks & photo 5 is him in his tank today.

It started as a tiny bump on his side, then it looks like the problem scale just came off one day (photo 2, right before it fell off. There was NOT a hole underneath, thats just a shadow from the light). And now it looks like this. The scales are flat now, but different colored, and it's now multiple scales that have a different color. Including some kind of brown-ish color to them?

Any idea what could be going on? He's otherwise perfectly healthy, in fact he's gotten bigger and his fins have grown like crazy since I got him. No signs of stress, ever.

Eats very well, almost exclusively live food (right now mostly mosquito larvae and daphnia).

Don't know the tank parameters, haven't checked recently. Every time I have in the past, everything is well within healthy range. Really not worried about his water quality with all the plants in there.

Tank is 45 liters (~12gal), kept at 26°.

He doesnt even seem to notice this, and it doesnt bother him at all. Never flashed against objects or anything like that.

Anyone seen something like this before? The fact that it keeps changing is what worries me.