What is the jargon term for something which is (unjustifiably) unique?

Probably you are thinking of (special) snowflake, which Wikipedia defines as:

Snowflake is a 2010s derogatory slang term for a person, implying that they have an inflated sense of uniqueness, an unwarranted sense of entitlement, or are overly-emotional, easily offended, and unable to deal with opposing opinions. Common usages include the terms special snowflake, Generation Snowflake, and snowflake as a politicized insult.

The related term special snowflake syndrome also matches the intended usage:

(derogatory) The conviction that one [...] is, in some way, special and should therefore be treated differently from others.

These are just general definitions, but the term is definitely used to describe software. For example:

Another way to make only the background transparent is by using a transparent png as background-image, and then use this jQuery fix for the special snowflake IE.

(Matteo Riva on Stack Overflow)

(It’s easy to find more examples.)

The reason “special snowflake” makes sense here is because software like this is unique for no good reason (“an inflated sense of uniqueness”) and often requires extra effort to deal with (“an unwarranted sense of entitlement”).