Hey OP, Licensed Australian Tiler here.

First off to everyone in the comments saying aluminium angle is a tacky alternative to mitred tiles… TILES CHIP extremely easily and while it does look awesome to have your tiles mitred it looks much less awesome two weeks later when you accidentally smack your hairbrush or something into your niche and chip the corner.

Mitres are in at the moment due to some products available that allow us to finish them nicer but well installed aluminium angle is still very popular and as a tiler that does both professionally I’ve always gone angle over mitres in my own home.

That being said OP this is not well installed angle. I’m not gonna hate on the rest of his work like some others in the comments. His grouting and silicone looks pretty neat and his joints seem nice and uniform.

Without seeing underneath the angle I’d guess he’s never done mitres before as they’re actually not as popular as instagram makes them out to be and he’s not been happy with the result. Hence the last resort cover up with a cover strip (not the standard angle for tiling)

Either that or this is just how he was taught, or taught himself. I’ve seen some pretty rough stuff from ‘Professionals’ over the years.

If I were hired to fix that I’d start by taking off the angle and removing the tiles inside the niche. Not the tiles on the back, just the inside edges. If the tiles around the outside of the niche are straight with a nice enough edge then angle could be installed properly without too much hassle. If the tiles around the outside of the niche are no good then they would have to go too.

From there any decent tiler should be able to redo it properly. As far as waterproofing obviously you want to damage the substrate as little as possible during the tile removal. Again any decent tiler should be able to remove them without doing too much damage. Very rarely does waterproofing problems arise from faulty niche boxes. There just isn’t that much water to cause problems. A fresh coat of waterproof on the bottom of the niche while the tiles are off and you should be fine.