Soldermask color relating to thickness

I'd guess ...

+ Pads have no solder mask, clear or otherwise.

+ Pads won't take solder, because you're trying to solder to nickel.

+ Nickel on the pads is oxidized and is sticking out its tongue at the flux you're using.

+ Something went south with you supplier's via tent process

What does your board fab say? Have you sectioned a board?

[it's always fun to blame the board fab for everything bad that happens in production. If that doesn't work, blame the knitters.]

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If you want to check if solder mask on tented via, better to do cross section to have a clear view.

Sometime supplier will not tell you the truth once you ask question for potential quality issue.

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