Thank you benwiggy for your detailed answer.
Sorry - how stupid of me, I am using Capitan 10.11. - Sibelius writes PDF 1.4 (Mountain Lion).
I also noticed writing in Sibelius EPS. With 200 pages of music score and TABs, this problem appears in one composition consisting of three voices. As long as there is a solution, everything is okay. It makes no difference to me to export EPS. I did this for decades.
Publisher, to be precise the whole Affinity Suite, surprises me in every way. Since I am switching to Catalina later this year, ending my 25-year love-hate relationship with Adobe (Suite 5.5.), I am not confident that Publisher and Designer are completely sufficient for my needs. However, there are still huge differences between Photoshop and Photo.