After considerably discussions and analysis, a second observation of Hume's Leaf Warbler (Phylloscopus humei) for Bulgaria was accepted. The bird has been observed in Varna - Primorski Park by Anett Oehmig on 12th Feb 2022: https://macaulaylibrary.org/asset/417042941
After that Rositsa Tsvetanova take photos of probably same bird, same place on 15th Feb.
Andrew Paul Bailey and Teodor Trifonov also take photos & video of the birds on 16th Febr. Their sonogram is from the recording uploaded on xeno-canto (https://xeno-canto.org/702708). Unfortunately, only those two calls could be recorded, but the other ones heard were identical.
Finally, the bird photos were taken by Zdenek Vaclav on 18th Feb.
Additional sonogram was prepared by Kalin Velev, helping for bird identification.