Marco I tested your example and got the result you described with both xvi and pdf outputs. I think you have found the transition defined by ‘vthreshold’.
From the Therion Book…
“ vthreshold <number> <units> ◃ threshold for interpreting LRUD readings as left-right-front-back reading perpendicular to the shot. If passeges are horizontal (inclination < vthreshold), LR is perpendicular to the shot and UD is vertical. If passages are more or less vertical (inclination > vthreshold), even UD becomes perpendicular to the shot – otherwise passages would not look very good. In the case of vertical shots, UD is interpreted as north-south dimension from the station to allow tube-like modelling of verticals “ I had thought the default value of vthreshold was 60 degrees, but it seems it may be around 67.5 degrees. Bruce