Maybe in the material settings is there any “setting” that say something like output alpha? because I had a similar issue with Post-Process and render Target and the issue was that setting alone. @Morrango seems to be correct for some technical reason in 5.1.0 you must use linear color instead of color (also down in material “stats” window it will notify you that color is not correct anymore).
My issue if that helps and how I solved it.