Course stops giving turn by turn navigation part way through?

The watch can only load so many course points; once it runs out it will stop giving turn-by-turn. Was it a very long route? How did you create it and how did you transfer it?

It was an 18 mile route, created via gmap-pedometer then imported into Connect courses, then transferred to watch via mobile Garmin Connect.  Is it a limit on loading the number of course points for the currently selected course or a limit based on the number of courses that have been downloaded to the watch, although only one is active at a time?  What is the limit?  Is there a way to get around it without chopping up the course?  Seems pretty useless to have course navigation that only works part way.  I'm not very thrilled with the latest Garmin experiences.  In the past, I've been very happy with Garmin, not quite as much lately unfortunately.

I'm not sure on the maximum for the 645. If you search the forum, you'll see anecdotes of people hitting the limit, e.g., 50 or 200 points for various devices, and I believe it's per course. But a warning should come up.

Relatively few people use the 645 (forum) and fewer still for navigation. There's more info in the forums for the 935, 945, and Fenixes that may help you. A different method of route creation or loading may help, or your original theory could be correct.

Maybe related. I always create routes in hiking apps (ViewRanger in my case), then export them into GC.

Routes are very detailed, because they automatically follow the openstreetmap traces. Last long hike the watch simply froze around 90% of the track, forcing me to reset it during the activity. Last year, same firmware, I recall it crashed almost at the end of the course.

So yes, maybe it has untold limits and in the best case it just stops following the course or, like in my case, it goes crazy.

I've hit this a few times with my 645 when doing a very long route with a lot of turns.  It's a 200 course point limit.  You should see a warning come up when you select the route on your watch.  It'll go through the 200 points then stop giving turns (but you can still see the route on the watch).  You can stop one route then start up another one without stopping your run.  Yes, it's choppy and annoying, but it's a limit of how much the software can run on the hardware the size of a watch. 

If course points is the problem, you may also be able to get around it by using a different route tool and copying the GPX file directly to the watch via USB, though not every tool will give you turn-by-turn. Again, if no one else chimes in, check the other forums.