The issue is that the instruction given is correct and at least a minute in future and Siri use to do it just fine until around 18.2.
If it’s 5:25 and a command for an automation is created to run at 5:26, the instruction is not correct as, by definition, there are fewer than 60 seconds until 5:26. Even at 5:25:01, there are only 59 seconds until 5:26:00, which is less than one minute.
Sounds to me like they found a bug and fixed it.