If the tracks are laid directly on the board, the 24922 derives a climb from the K track rail top to the C track rail top. There’s a general recommendation not to have turnouts near a climb, as that combo could cause derailments. I can’t see why the BR218 should be more sensitive than other engines, maybe this content will appear in all new manuals?

The problem is easy eliminated if e.g. having track bed under the K tracks.

I can’t believe the different rail profile/code between K and C could be a reason for the instruction.

I have a newer Marklin br 218 Cottbus version. Same manual. No problem negotiating any curves or turnouts. The only complaint I have with it is the engine turbo sound cuts off sharp on stopping at times. Depending on how quickly you stop it. Otherwise is a gradual wind down to idle. Maybe yours will have the improved sound project files.
The coupler pocket is very difficult to remove and changing the plow to the optional one doesn't allow room for the tiny torx screws to line up with the holes in the frame.