It looks like regression, but maybe it is a different root cause.

Consider this parameter in a template:

data "coder_parameter" "compute_instances" {
  name        = "Compute instances"
  icon        = "/icon/rubymine.svg"
  type        = "number"
  description = "Let's set the expected number of instances."
  default     = "3"
  mutable     = true
  validation {
    min       = 2
    max       = 5
    monotonic = "increasing"
  }
}

1. User starts a workspace with compute_instances = 3.
2. User upgrades the workspace to compute_instances = 4.
3. User upgrades the workspace to compute_instances = 3

UI does not inform about the build problem, but should. If you inspect the console, you will see HTTP 400.