We have a flight plan created to fly around the Idaho backcountry.  When xxported as MSFS 2020 PLN file, it won't load in MSFS 2020.  When the same flight plan is exported as a FSX/P3D PLN file, it loads just fine in MSFS 2020 - which we find to be very odd.

Here is dropbox link to all three versions.

LNMPLN https://www.dropbox.com/s/3uepvwvwo8gdo4b/River of No Return LNMPLN.lnmpln?dl=0

FSX/P3D Export https://www.dropbox.com/s/tn7brhtndyepcag/River of No Return FSXP3D Export.pln?dl=0

MSFS 2020 Export https://www.dropbox.com/s/0qad8bnosdaja26/River of No Return MSFS Export.pln?dl=0

This custom scenery is part of the flight plan - but having custom scenery doesn't explain why the FSX/P3D export works in MSFS while the MSFS export of the same flight plan will not load in MSFS 2020.

flightsim.to
https://flightsim.to/file/9140/cougar-basin-pz12-idaho-playground
https://flightsim.to/file/8791/lake-mountain-idaho-a-stol-paradise
https://flightsim.to/file/8849/buck-lake-land-and-lake-retreat-idaho
https://flightsim.to/file/1156/bush-strips-in-idaho

Bush League Legends Scenery
https://mega.nz/file/Dx4y0aZA#4kUYrixW6YUpeTAvjZlQuG2txNwi2oftdNSIbqNKlyk
https://mega.nz/file/G54WHAwb#2ZQdecuoDOL0Q_wmEd6MiWmzyWi1GZyMKPalJTxEdvc

Thank you for all the files but I cannot see any obvious issues in the PLN.
There are even no procedures involved which is usually a cause for trouble when loading plans in MSFS.
Maybe MSFS cannot deal with some of the add-on airports?

Flight plan loading is quite a mess in MSFS and much more reliable in FSX, P3D and X-Plane.

Alex