Assalaamu alaykum,
In a PoS model, you only get selected to be a node in the network if you have a specific amount of coins stored previously in your wallet. After being a node, if you want to be qualified for being a miner you will need to deposit a certain amount of coin, after that there will be a voting system for choosing the validators. When it’s all done, the miners will stake the minimum amount required for the special wallet staking. New blocks will get created proportional to the number of coins based on the wallet. For example, if you own 10% of all the coins, then you get to mine 10% new blocks.
There is no issue in this from a Fiqh perspective as one’s coins are not gambled. One’s holdings are used to determine who gets to mine the new blocks. The rewards do not come from others staking their coins either, but from the transaction fee.
Allah knows best