This is a repo for this corresponding issue: simplecov-ruby/simplecov#920
In simplecov 0.19.0 there was a change that SimpleCov::Report.from_hash does not return one result, but an array of results.
This broke our coverage merger.
Similar to what steranking found out, I also found that .resultset.json NEEDS to have absolute paths:
simplecov-ruby/simplecov#920 (comment)
My situation was, that I tested the merger - see custom-merger.rb - locally where the absolute paths are different from our CI run.
CI - /app/app..., local - /User/name/app/app...
Please see more details and communication in the linked ticket above.