Das Onlinelexikon erklärt auch, in welchem Kontext eine Redewendung benutzt wird.

The preposition in can take either dative or accusative:

• Dative = location / state (where something is)
• Accusative = direction / movement (where something is going)

Here, Kontext is not a direction but a situation or background in which something happens, so it’s a static location in an abstract sense: in which context → dative.

• Kontext is masculine.
• Masculine dative singular of welcher = welchem.
• Masculine dative singular of Kontext stays Kontext (no extra ending).

So:

• in welchem Kontext = in which context (dative → location)
• in welchen Kontext = into which context (accusative → movement, would sound wrong here)