Update: So the IRS self corrected the issue and sent me a letter a couple weeks after my payemtn saying they applied the 2018 payment to my 2021 balance. So the issue is corrected. If you try to login to your irs account and make more payment it STILL tries to apply ot to 2018 for some reason, so, I've found a way around this error. Instead of logging in, just choose to make a GUEST PAYMENT on the irs site, type in your info, and it will let you specify the exact year . That's the only way I've been able to make payments for 2021 lately!
