Definitely water damage. If it is next to a shower, being it is the floor wall intersection my gut says shower leaking. Show a picture from further back.

Rot.

@zumi rot from water damage?

@millworkman I will get picture

A zoomed out picture will help but if I am seeing this correctly: it looks like the edge metal at the tile stops short and was filled with a huge chunk of caulk, which has failed. I can't imagine what else would cause wood to rot like that aside from water damage. Does this room happen to be above an unfinished basement where you can look at the underside of the floor?

Definitely water leak and yes, it does look like rot. Its been leaking a long time, to get that bad.

100% a water leak. Shower is always the most likely culprit but could be the toilet leaking under the floor and wicking up. But probably the shower.

@cat_ky

Here are the photos from further out

This photo you can see even the molding behind the toilet there's stains (on right side of photo marked in red)

Left side of toilet

left side of toilet

Left side of toilet

right side of toilet

From inside shower

I do not see a compromise from inside the shower

I'm wondering if it's maybe a leak from the toilet and going under the flooring (it is luxury vinyl flooring)

Thank you

AMB- this is first floor (there is no basement)

Still betting on the shower. How about pics of the entire shower, in and out. You would not necessarily see where it is compromised,

To some extent it doesn’t matter. You have to pull out the toilet, the flooring, and the baseboard regardless. When you pull up the floor you will probably be able to see where the leak is coming from.

" Plus improperly waterproofed walls. The inside of the shower shows improper waterproofing for the pan. "

I agree it is most likely the shower but where do you see the above @Zumi.

There is a bulge in the wall, below the picture of the words right side of toilet. If it was the toilet, it would not have caused damage there, and that bulge, appears to be from moisture. In any case, buy a new wax ring for the toilet, and remove the toilet and check to see if it is leaking. Its not a difficult job, and can change the wax ring, and have it back down, in 10 minutes, once you check to make sure its dry. All that white markings on the floor are also water damage, so one way or another the toilet and flooring need to be removed anyway.

We had a leak from a broken toilet flange and only one wax ring installed on tile. We had water damage on the wall behind and to the ride side of the toilet and baseboards.

Even if it isn't the toilet, the caulk around the bottom of the toilet looks awful too, so that should also be replaced.