https://plantuml.com/faq#e216d554ed154ada
Please read the "I don't like how the diagrams generated by PlantUML look" section.
PlantUML does the layout for you, so you loose control of that, but gain the ease of use, and the ability to compare versions textually (the ability to have diagrams next to code, that you can get history on it huge IMHO)
I often will generate a diagram and hand it off to the art types to make it look pretty.
If you add/change/remove an item, PlantUML might have to layout your entire diagram differently (something you still have to do if you are maintaining one by hand), and this can make things look different. The difference is with PlantUML that process is done for you just by re drawing your diagram... by hand one change can mean days of work.
If that level of control is something you need, I suggest using other software (Inkscape is good for professional looking diagrams IMHO).