By no means have a found a solution to this strange problem. I won't be surprised if it randomly stops working again. I have seen some strange code behavior in my day, but this one ranks as one of the strangest.

This last bit of testing I did not enable debugging because I am using a newly created user for running the tests the a Community profile. The user only exists in the context of the test.

But I now see that when I ran my Developer Console snippet, Salesforce automatically created a 30 mins debug session for me. So my original theory is correct, the code responds differently (specifically List.contains) when being run under the context as a debugged user. To prove this, I deleted the DEBUG session for my login, and the tests failed.

It's been weeks since posting this problem, and Salesforce can't help and I took another look at it today. I painstakingly debugged a user using System.assert in unit tests I wrote for it and what I saw was astounding.

I found that Salesforce would inconsistently return false on List.contains() when I proved that the List contained the items I was looking for. Here are actual debug printouts from Salesforce when it was running false:

selectedSkills[0].Skill__c: a0c1F000000IZ0rQAG

skills[0].Id: a0c1F000000IZ0rQAG
skills1.Id: a0c1F000000IZ0sQAG

selectedSkillIds[0]: a0c1F000000IZ0rQAG

selectedSkillIds.contains('a0c1F000000IZ0rQAG'): false selectedSkillIds.contains('a0c1F000000IZ0sQAG'): false

I bolded the matching strings. Even though selectedSkillIds contained a0c1F000000IZ0rQAG it still returned false.

This is where it gets weird. I recreated a simple contains test in Developer Console, and the contains method worked correctly.

I reran my test again and amazingly, the contains method worked correctly in the test. I made no changes to the code/login and I got different results. The only thing I did was run a code snippet in Developer Console to very List.contains was working as I expect.

Here's the output again with it working (again, bolded where matched):

selectedSkills[0].Skill__c: a0c1F000000IZ1GQAW

skills[0].Id: a0c1F000000IZ1GQAW
skills1.Id: a0c1F000000IZ1HQAW

selectedSkillIds[0]: a0c1F000000IZ1GQAW

selectedSkillIds.contains('a0c1F000000IZ1GQAW'): true selectedSkillIds.contains('a0c1F000000IZ1HQAW'): false

By no means have a found a solution to this strange problem. I won't be surprised if it randomly stops working again. I have seen some strange code behavior in my day, but this one ranks as one of the strangest.