There’s also a longer article that explains the background by BBC:

EU backs indefinite freeze on Russia’s frozen cash ahead of loan plan for Ukraine

Context:

• it’s easy to guess that Russia owes Ukraine reparations for agression
• using Russian assets as a loan (or a guarantee to a loan) to help Ukraine is politically reasonable
• however, it is legally tricky as the EU could some day fail to keep the assets frozen
• article 122 of the EU founding treaties allows for qualified majority decisions instead of consensus if there’s an emergency situation threatening the economy of the EU or one of the member states
• a condition that funds remain frozen as long as Russia threatens the EU seems to result in a long freeze, unless Russia actually changes its policies

As for the concern about markets, I think it’s exaggerated. Nobody in their right mind is expecting to keep their assets in foreign banks if they pursue a war of agression. A reasonable party to a conflict should expect their assets to be frozen and seized much faster than it’s taken.

Also, this seems to reliably remove Russian incentive to threaten or persuade politicians in Belgium. If they no longer hold the keys, harming them won’t get anyone any goods.