Having the same problem here - I initially created a complex flow including an Approvals step under "My Flows", then added it to a Solution, where it continued to work. The purpose of moving to a Solution was so that I could break it down into multiple smaller flows with "Run a child flow" links connecting them.
Bug #1 - The original one can't be used as a child flow, because: "Flow save failed with code 'WorkflowUnderWrongSubscription' ...", and you can't see or change the subscription for either flow, and it doesn't update like it presumably should when adding that original to the Solution.
Ok, create a new Flow inside the Solution, so that it gets the right subscription setting, then copy-paste the components across.
Bug #2 (this one)- Approvals component says "Invalid connection" after being copied in, with no way to fix it or create a new one inside that Flow ("Create simple connection failed") .
Presumably the connection is dependent in some way on this subscription setting and only works while that still has the value carried over from "My Flows". Once inside the Solution it will still work, but new Flows you create inside the Solution simply can't use Approvals actions. My grand idea of simplifying this by using "Run a child flow" seems dead in the water now (only works in Solutions), and I'm sure not paying MS for this garbage just so that I can use HTTP parent-child integration instead.
