Japan 46 Campaign Supply Sources

[Indragnir]

The regiment is a dummy organization without an HQ, similar to that used for the artillery units. That shouldn't be causing the issue.

The flags and unit type for Japanese and US mortar units are identical, so it's not an OOB unit setting.

Changing the organizational unit size of the mortars or the regiment doesn't solve the issue.

As to OOB errors:

-The Japanese 93rd Division's Recon Battalion has a replacement rate of 93%.
-The Japanese 201st Division's 501st Infantry Regiment has a replacement rate of 501%.

Mortar issue is a known bug of gold version. Any time you put a mortar unit into a "unit folder" in the OOB that unit can only use other units in that unit folder in the OOB as spotter. If you put the mortars directly under the unit main structure they work as intended.
That won't work:
Divisional HQ
[Divisional Units]
Martar Regt
         Mortar A/Company
         Mortar B/Company
         Mortar C/Company

That will work:
Divisional HQ
[Divisional Units]
Mortar A/Company
Mortar B/Company
Mortar C/Company.

Hope I explain myself.