I must apologize, because it was allready written here somewhere, but I cannot find the topic. O:-)
If I edit the first post of a thread, the topicon will not stay the same as before, there will always appear the xx.png
Confirmed and tracked with steps to reproduce:
https://github.com/elkarte/Elkarte/issues/1820
And just because I was thinking to go bed, quick fix here:
https://github.com/emanuele45/Dialogo/commit/3af0d7602a251cef76652a48020ba62a238a809b