Bread – Is the biga supposed to be liquid

this is a technique that might help with the shell issue:

I suspect your croissants are losing humidity during proofing, possibly freezing the butter block, and possibly baking resulting in a harder shell. If you have a humidity controlled proofer and oven, then those parts may not be your problem. Otherwise, read on:

Peter Reinhart in The Bread Baker's Apprentice suggests pouring boiling water in a flat pan/container and placing in the bottom of the oven during the baking (of breads) in order to keep up the humidity. This Link has the recipe published with Peter's permission. Notice how the freezer doesn't come into play (freezer is a dehydrator and pretty good at it, too) although some recipes call the freezing part a must.

Croissants are lighter than bread loafs so they may require a different humidity profile than breads during baking (as noted by commenters). However, for proofing humidity levels should be high and the temperature around 80F or 27C.

To achieve that, for the proofing period keep the dough in a turned-off-oven and turn on the oven light a long with a the boiling water method. This usually prevents 'the skin' formation and a dried out crust. It also helps with getting a better rise during baking. The oven light bumps up the temperature but if you're room temperature is close, don't bother.

You may wish to read this piece on humidity and baking and generally the Maillard Reaction. Here's an excerpt that might explain the problem:

"Under dry oven conditions, the dough will have rapid evaporation of the water from
the exposed surface. This will cause premature formation of a dry inelastic outer shell
on the bread surface"

There are three major things that will cause collapsed yeast raised breads:

• Underdeveloped gluten (possibly from insufficient kneading), which doesn't create enough of the gluten network to hold the gas from the yeast, especially during oven spring. This may be exacerbated by using low-protein flours, which do not generate as much gluten as high-protein flours (called strong flours in Europe). This tends not to manifest as a collapse, so much, as a failure to rise.

• Considerable overkneading, to the point where the gluten network begins to break down, and cannot hold the gas (this is quite unlikely, unless machine kneaded)

• Over-proofing (allowing to rise or ferment too long), which creates more gas than the gluten cells can hold, leading to their rupture and collapse when the heat of the oven causes expansion and oven spring. This leads to collapse of the loaf.

Without more information, it is difficult to tell which of these you are experiencing.