Hyphenation of a compound modifier formed of an adjective and a noun

Earlier questions on the hyphenation of compound modifiers have been well answered, so now I would sharpen the question.

We seem to agree that this has good hyphenation:

The question is well answered. The well-answered question
illuminates its subject.

So, if well answered precedes the noun, it gets a hyphen. If it follows the linking verb, it does not get a hyphen.

So far, so good, but now try this:

The software is open source. Open-source software is distributed with few restrictions.

The "is open source" does not look quite right, does it? Or am I just looking at it the wrong way? (I admit that it does not look too bad, but is it right?)

Does it matter that answered is a participle, whereas source is a noun? Or that well is an adverb, whereas open is an adjective?

You and I could evade the question by writing "is openly sourced," of course, but that's just wrong, and even if not so, the question as asked would still want an answer. My question regards the principle of the thing.

When should I hyphenate a compound like open source, used as a modifier?

Answer

"The software is open source. Open-source software is distributed with few restrictions."

While I agree with you that "The software is open source" doesn't look quite right yet, opensource.org uses it exactly as you have above:

"Introduction: Open source doesn't just mean access to the source code. The distribution terms of open-source software must comply with the following criteria," etc.

From: https://opensource.org/osd-annotated

Over time, I think we will come to accept "open source" as used in your first example.