What transpired between photos 1 and 5? Time? Do you have some old photos of the powdercoated bike that might show the detail after powdercoating, but before riding?
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Photo 5 seems to indicate a slender seatstay cap with a smooth transition into the ST/TT braze.

In Photo 1, the seatstay cap appears to be fat and bulbous at the top, with a jagged edge.

What I'd do is first to take calipers and measure the width of the top part of the seatstay cap on the right and left (best you can estimate).

I'd guess that the left side will measure 1 to 2mm wider than the right side, which could be due to a glob of extra powdercoat.

Next, I'd take a knife and see if I could trim off a chunk of that part sticking out, or at least probe it to determine if it is plastic, brass, or steel.

Does your bike have patent vent holes? If so, another option would be to flip the bike upside-down, and squirt some light oil into the vent hole (colored?). I think the seatstay caps are at least partly hollow. If any oil runs out, you could have problems. A negative result would neither confirm or disprove what happened, but would at least support the conclusion that it is OK.

My interpretation is that this could be an extra glob of powder from when it was originally powdercoated, and not a serious issue.

Anything but powder, or signs that it has changed over time would be more worrysome to me.