Cloudwatch metric filter sees event, but alarm doesn't fire

I have set up a Cloudwatch Metric to watch a log file:

resource "aws_cloudwatch_log_metric_filter" "log_errors" {
  name = "${local.fullname}-log-errors"
  log_group_name = "/aws/lambda/${local.fullname}"
  pattern = "{ $._logLevel = \"error\" }"
  metric_transformation {
    name = "${local.fullname}-error-count"
    namespace = "MyApp"
    value     = "1"
  }
}

I can see the metric is working - note the dot at 13:15 below (me manually creating a log entry to test):

And an alarm to fire if the metric reports 1 or more events within a minute:

resource "aws_cloudwatch_metric_alarm" "log_errors_alarm" {
  alarm_name        = "${local.fullname}-log-errors"
  alarm_description = "log.error() count for MyApp lambda ${local.fullname}"
  metric_name         = "${local.fullname}-error-count"
  threshold           = "0"
  statistic           = "Sum"
  unit                = "Count"
  comparison_operator = "GreaterThanThreshold"
  datapoints_to_alarm = "1"
  evaluation_periods  = "1"
  period    = "60"
  namespace = "MyApp"
  treat_missing_data = "notBreaching"
  alarm_actions      = [data.aws_ssm_parameter.sns_topic_arn.value]
  ok_actions         = [data.aws_ssm_parameter.sns_topic_arn.value]
}

But despite the metric having an event (per above) the alarm is never fired:

I'm unsure how to debug this, as all the AWS resources are created successfully, errors that I create manually are passed to the metric, and I'm using a very similar alarm config in other lambdas successfully, where it throws alarms.

Why is my metric working but my alarm not alarming?

I'd put my money on the Unit being inconsistent between the metric_alarm and metric_filter.

You're setting the unit on the metric_alarm to be Count, but you're not setting a unit on the metric_filter's metric_transformation, so the metric_transformation will default to None.

Try setting the unit in the alarm to None or removing unit altogether.