Javascript – Array.splice in function sometimes not working as expected

I have some logic that, rarely – but on occasion, produces erroneous results, and I’m trying to understand why. I’m thinking that it’s likely the case that it’s a timing/order of operations issue.

The function/logic in question looks like this:

  async getAuths(pId, discipline, date) {
    const response = await Promise.all([
      this.db.Authorizations.getByPatientDiscipline(pId, discipline),
      this.db.Patients.getCurrentCovByPatient(pId, date)
    ]);

    const auths = response[0];
    const coverages = response[1];

    for (let i in auths) {
      const auth = auths[i];
      auth.coverage = false;

      // Load coverage if auth has one
      if (auth.coverageId) {
        const covgRes = await this.db.Patients.getByPatientPayer(auth.coverageId);

        // Apply coverage if found
        if (Array.isArray(covgRes) && covgRes.length > 0) {
          auth.coverage = covgRes[0];
        }
      }

      // Todo: Translate to use Array.filter to simplify code
      // Take out coverages with authorizations
      for (let j = coverages.length - 1; j >= 0; j--) {
        if (
          coverages[j].id === auth.coverageId &&
          moment(auth.start, format).isSameOrBefore(moment(date).format(format), "day") &&
          moment(auth.finish, format).isSameOrAfter(moment(date).format(format), "day")
        ) {
          coverages.splice(j, 1);
          break;
        }
      }
    }

    console.log("coverages - withoutAuths 231: ", coverages);

    return {
      authorizations: auths,
      coveragesWithoutAuths: coverages
    };
  }

You’ll notice that I am logging to the console any results remaining in the "coverages" array on line 231. At that point, because coverages WITH authorizations have already been removed in the block above, this array should only contain results that DO NOT have authorizations. However, on occasion I see an object still sitting in that array on line 231 that I know DOES have a valid authorization. So it seems that the logic taking out those elements in the array that do have authorizations is — on occasion — not working, again, perhaps a timing/order of operations issue, or a situation where one object in an array is being polluted by another object in the array.

I’d appreciate any insight as to what the issue may be here.