Software 21.17 for explore 2

Thank you. Does this only apply to highways and major roads?

Tried, doesn't work with the OSM downloaded from https://alternativaslibres.org/en/downloads.php. Does it work with OpenFietsMap?

Maybe you can suggest what exactly should be changed here to remove those black lines?

Have you edited the typ files?

Yes, replaced the typ file using the link provided by bikepc01

I understand that the issue also affects Garmin's own maps on the affected gps units.

I know nothing about how any of this works. I just use the hardware. I am in awe of the people who can program and work out what has been done to screw things up and come up with a solution.

@Yuri, you'll have to edit the typ-file. Change all polylines from vector-type to bitmap-type. Quite a lot of work! For the long discussion about this subject, see: https://forums.garmin.com/sports-fitness/cycling/f/edge-1040-series/365387/third-party-maps-on-new-update#pifragment-1290=3 , an important how-to example is from @Speichenkarte

After you have created the modified typ-file you need a tool like JaVaWa's TYPchanger to replace the existing typfile with the new bitmap-only version.

Applies also to cycleways not only highways and major roads.
What third party map are you using and where do you download it from? Can you give a link?

Thank you for the instructions. Turns out I forgot to edit it.

For people who use the same OSM downloaded at alternativaslibres.org, I've emailed the website Admin (Carlos Dávila). There is an extra step: "Note TYP files have an internal number (FID) which is unique and must match the one of the gmapsupp.img." And he sent me a modified TYP file (different for each map), works great!

The only disadvantage by using bitmap-type is the display loses antialiasing function.