Hi! I had a look at OSM (as I'm sure you have) and didn't spot errors or changes in the last couple of months that would explain this (I could have missed something).
I tried to replicate your problem using car routing like this:

but it didn't happen. (Android OsmAnd+ 3.7.2, England map 1 May 2020.) I also tried cycling, with and without elevation.
But my own observation of poor routing recently has been for walking when a bridleway should be used but isn't. I thought I'd try it at this location, and sure enough:

The bridleway (in green, this is Touring View) should have been used but wasn't, with or without elevation.
Then I thought I'd place an intermediate destination part way down the bridleway, and the turn-by-turn directions go haywire!

I have no idea what's going on! I hope somehow this helps...