I found out I used the insertion in a different way than what you did. After tapping + you get the marker sentence “select a location in the map”. I follow that saying literally, without tapping that sentence first, so I don’t get a red marker on the screen. If I do it the way WindyMaps expect me to do, it works perfectly. So now you probably can replicate what I did “wrong”.
Having said that, I find tapping the marker sentence a superfluous and therefore inefficient action. Why would you need it if the app can find out that by itself easily, it knows you want an insertion, so why a tap ‘n hold not enough?
By the way, I have/had some other route planners (among Garmin), but I’m switching to WindyMaps. I just would like to see the app becoming more efficient in some aspects (see my suggestions, in which the waypoint insertion does not make part yet), so that it becomes at least equal to or better than other route planner tools.
I would be really surprised, having seen the wonderful Windy app, that the Windy team wouldn’t be thinking of making it as efficient as possible... ;o). I’m switching to Windy because one can feel the tremendous drive behind it.