This issue is not related to the maps, but to the firmware of the Epix Gen 2 models (including the Pro models), ForeRunner 965 and related AMOLED display Garmin watches.

It is a transient rendering issue, which is most quickly fixed by simply rebooting the watch. The issue is seen on ALL maps, including the pre-loaded Topoactive Garmin maps... It doesn't happen often, and is not causing any issues with the maps themselves; it appears to be a rendering engine issue in the firmware

A number of threads have shown up on the Garmin forums, and on the many Facebook groups for Epix watches; including photos of the map issues.

Here's a link to just one of the Garmin forum threads about it: https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/epix-2/335693/epix-pro-corrupt-map

Below are a couple of screen shots of the issue showing up on an Epix Gen 2 Pro model.

Garmin are aware of the issue and they state "Our engineering team continues to work on resolving the issue as soon as possible."

Please Note: I have an Epix Gen 2 Pro 51mm an duse my own maps on it, and I have never personally experienced this issue.