Custom subscription in appsync: request exceeds filter attribute limit

I've a custom subscription in appsync, that used to work a few days ago (no change there since then!). But now I get an error back when subscribing to it: the current request exceeds filter attribute limit 5

But there are only 2 filter attributes in my variables for the subscription query: "filter": "{\"testId\":{\"eq\":\"123\"},\"type\":{\"eq\":\"event\"}}",

If I omit the testId attribute, it's working fine.

Did someone faced a similar issue?