Using LPI w/ tented vias

I need some information regarding the acceptability/unacceptability of using tented vias with LPI soldermask.

Long story on all involved, but basically I have a board that I have had manufactured with no problems which has BGA/uBGA parts, tented vias, and using LPI. The responsibility for having the boards manufactured has recently been transferred to a contractor who is basically refusing to have the boards manufactured unless I approve a change from tented to plugged vias. A quote from them is "It is generally unacceptable to tent vias with a liquid mask".

I have nothing concrete with which to argue with them (any good references would be appreciated). Any opinions as to what I should do? (Changing contractors is out of my control). If I change to plugged vias, I am concerned with the reliability of the BGA assembly? Is there another viable soldermask alternative?

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