Quick answer: Unreal material pin types changing between engine versions, breaking node connections? Some pins moved from float to half; rewire manually or run the migration tool.
UE 5.3 -> 5.4 upgrade. Half a dozen material graphs show broken pins despite no code changes.
Run material migration tool
Tools > Material > Migrate Materials. Walks every material; rebuilds pin connections per the new types.
Manually rewire
For one-off graphs, open and fix. Tedious; works.
Pin engine version in CI
Lock the engine version. Surprise upgrades cause this class of bug; pinning forces deliberate upgrades.
“Engine upgrades evolve types. Type evolutions are migrations.”
After every engine upgrade, dedicate a sprint to material audit. The bugs cluster; one sweep catches most.