Is this the same magic that means microsystems has to take a MtM writedown every time its bitcoin tanks, but cannot do so when it goes up? xD

I am generally pretty pro expecting the MtMing of liquid assets, but very sympathetic to areas where hard to price illiquids are unpleasant

The accounting of "cryptocurrency" is still being developed, but, yes, Bitcoin holdings take an impairment when their value drops below a certain point. This doesn't typically affect cost basis. It may be deducted as losses on a tax return but that might impact cost basis. I've heard from other accountants (a year or so ago, so take this with a lump of salt) that some companies are trying to account for cryptocurrency as inventory, as a "hot asset," as cash equivalents, or as intangibles.