I think there are some XIX century frescoes on the medieval tower of Rila Monastery, Bulgaria.
My understanding is that "Hrelja Tower" is the ONLY medieval part of Rila Monastery remaining - the main reconstructed part contains "contemporary" 19th C frescoes (which don't therefore meet a criterion of being in a Medieval building) but Wiki (and elsewhere) says of the tower - "The stone tower is 23 metres (75 ft) high and has an almost square foundation. There are five stories, not counting the cellar, with a chapel devoted to the Transfiguration of Christ on the top floor; the chapel features fragments of 14th-century frescoes. The tower has a single entrance on the first floor, at the time probably reached through a ladder; from the entrance, the chapel can be accessed using the stone stairs built into the walls. It is thought that Hrelja's Tower was used as a protection for the monks, as well as a cache for valuables, a jail or a place to isolate mentally-ill people. A two-storey belfry was attached to the tower in 1844"
So those frescoes don't meet the criterion either.
Would you count the Wartburg as being "Medieval"? It has 19th C frescoes. "the Sängersaal (with frescoes of the Sängerkrieg by Moritz von Schwind) and the Festssaal on the top floor. The latter also features frescoes by Schwind (on the triumph of Christianity) and served as the inspiration for the Sängerhalle at Neuschwanstein Castle. The ...... . None of the wallpaintings, including those in the Landgrafenzimmer or the Elisabethengalerie, are actually medieval in origin, but were created in the 19th century"
I am bit with Els on this - how much "medieval" has to be left to count.