When merging multiple coverage files (e.g. from the temp-dir when generating reports or running the merge command) in getCoverageMapFromAllCoverageFiles(...), processing of individual files is parallelized and uses multiple threads:

However, the current implementation causes multiple threads to potentially call map.merge(...) concurrently, leading to undesired side effects unless the merge function is implemented in a thread safe manner.

The used p-map package and in particular it's "sister package" p-map-series also seem to document that this is not a safe usage, as the separate p-map-series package states in its documentation:

Useful as a side-effect mapper. Use p-map if you don't need side-effects, as it's concurrent.

=> p-map on its own is also not intended to be used with side-effects.

I can't provide a working example demonstrating failure of this snippet at the moment as I've just stumbled across this whilst working on something else but fortunately did not run into a specific bug due to this (yet) - as this is about a race condition it would be difficult to reproduce anyway.