Currently using Reaper v6.21 - January 23 2021 on Windows 10, fully updated.
When exporting a set of regions by right clicking in the Region/Marker Manager, regions with start and finish times greater than one hour are exported with the format hh:mm:ss.decimal. If reimported, the hour component in these fields is treated as minutes, minutes as seconds, etc.
I've attached an example project with one region defined a couple of hours along the timeline. There are two screen grabs of the Region/Marker Manager. The first shows the region list as defined in the project, the second after re-importing the a region list exported from the project.
I've found that I can get round this problem by manually editing the CSV file to just include minutes. i.e. A time of 1 hour 20 minutes and 4 seconds should be written as 80:04.000.
I hope this is easily fixed, I'm actually using this feaure regularly to import an edit lists for a set of weekly Zoom sessions