MPI timeout (?) sometimes in Izumi nag tests

### Brief summary of bug

Some Izumi nag tests sometimes fail in the run phase with `Warning: Floating underflow occurred` in `cesm.log`. Re-submitting usually fixes it.

### General bug information

**CTSM version you are using:** ctsm5.3.002 (but this has happened to me before, not just with this tag)

**Does this bug cause significantly incorrect results in the model's science?** No?

**Configurations affected:** Izumi nag

### Details of bug

Affected tests (today, that is):
```
ERI_D_Ld9_P48x1.f10_f10_mg37.I2000Clm50Sp.izumi_nag.clm-reduceOutput
ERP_D_Ld9.f10_f10_mg37.I1850Clm60BgcCrop.izumi_nag.clm-clm60cam7LndTuningModeLDust
ERS_D_Ld15.f45_f45_mg37.I2000Clm50FatesRs.izumi_nag.clm-FatesColdTwoStream
SMS_D_Ld5.f45_f45_mg37.I2000Clm60Fates.izumi_nag.clm-FatesCold
```

Unfortunately there's no useful traceback, so I'm not sure what's going on. However, it always happens _after_ the `CTSM: end of main integration loop` message is printed in `lnd.log`.