I added this to Customize Exterior's manifest, and it would no longer load, since my development environment does not have Seasonal Immersion installed.

  "Dependencies": [
    {
      "UniqueID": "Entoarox.SeasonalImmersion",
      "IsRequired": false,
    }
  ]

@spacechase0 It seems to be working fine. Here's what I tried:

1. Installed SMAPI 2.1.1 and Customize Exterior 1.0.3.
2. Changed Customize Exterior's manifest.json to this:

   {
     "Name": "Customize Exterior",
     "Author": "spacechase0",
     "Version": "1.0.3",
     "Description": "Change the exterior of your buildings.",
     "UniqueID": "spacechase0.CustomizeExterior",
     "PerSaveConfigs": false,
     "EntryDll": "CustomizeExterior.dll",
     "Dependencies": [
       {
         "UniqueID": "Entoarox.SeasonalImmersion",
         "IsRequired": false
       }
     ]
   }

3. Launched the game. Customize Exterior was loaded correctly:

   

4. Changed IsRequired to true.
5. Launched the game. Customize Exterior failed as expected:

   

Can you try it again? If it's still not working, please zip the mod folder you have.

Closed per discussion on Discord (working for @spacechase0 now).

Reopened to look into it further, since it's happening for spacechase0 again.

@spacechase0 I still can't reproduce this with SMAPI 2.3 and Customize Exterior 1.0.3, with its manifest.json changed to this (see log):

{
  "Name": "Customize Exterior",
  "Author": "spacechase0",
  "Version": {
    "MajorVersion": 1,
    "MinorVersion": 0,
    "PatchVersion": 3,
    "Build": ""
  },
  "Description": "Change the exterior of your buildings.",
  "UniqueID": "spacechase0.CustomizeExterior",
  "PerSaveConfigs": false,
  "EntryDll": "CustomizeExterior.dll",
  "Dependencies": [
    {
      "UniqueID": "Entoarox.SeasonalImmersion",
      "IsRequired": false,
    }
  ]
}

If it still happens for you, please attach your SMAPI log and a zip of your Mods folder.