the IRS should have known about that $15 in interest they paid you in 2020 when they sent you that refund. so I would not file an amended federal return.
you gave no info about your original filing. but likely when originally filed you received too much in PTC. so under the tax laws at that time you had to repay that excess which reduced your refund or resulted in a balance due which you paid. the tax laws were changed retroactively by the ARP act so that if you received too much PTC in 2020 you no longer had to repay that amount. for that reason, the IRS issued you the refund and should have taken into a/c the $15 in interest paid to you in 2020.
since you didn't mention what state, can not tell you how the state would treat that refund of excess PTC. it should not be income unless you deducted for state purposes the premiums you paid. the $15 would be taxable for state purposes in the year received
