In this case, if the graduating class right after you got a big leg up, that means they're more competitive and coming for the same jobs / opportunities as you. 2020 grads were competing with (and still are) the same new grad jobs as the 2021 grads with the pandemic. College loan forgiveness means those 3-5-10 years behind you now have a big cash leg up on you with years less work and investment. This $1,000 check at 18 means the 19 year olds may very well fall behind as those younger roll over them.
I'm not saying all these concerns come to fruition, but a leg up to your peers can have negative ramifications for you, even if the aggregate and future prospects for the aggregate is good.
Economically, inflation is equivalent to a tax on dollar-denominated held wealth, though. So it basically amounts to the same thing as taxes, but is paid for by bondholders, pensioners, people with fat savings accounts, and folks who lack bargaining power in their wages (i.e. most service & manufacturing workers) rather than high-earners.
Planting a tree is an investment in the future. Paying someone to dig up a tree and replant it in your yard is not.
If I were tweaking the program, I'd make it so that you could withdraw 5% of the value of your savings account each year from 18-22, 10% from 22-25, and get the balance at 25. Give people a chance to learn how the real world works and get scammed with lower dollar values before dumping all this money on them. I'd also consider just making it an outright grant at the time the money goes to the person, rather than a savings account with an accruing balance. This cuts down on the amount of graft that the financial industry can manage, makes the government expenditures more predictable, and eliminates potential injustices based on how well the money is invested.
In principle, just giving new adults the money directly to light on fire as they see fit shouldn't be inflationary.