In Linux Mint I have previously been able to disable Dropbox starting on system startup but now it actively fights this.
I can deselect Dropbox in Control Centre > Startup Applications. Then it won't start on system startup but as soon as I manually run Dropbox it immediately reactivates the Dropbox setting in Control Centre > Startup Applications.
The tick box in Dropbox's Preferences has no effect. This tick box has been inactive for some time (many months) but I have ignored this in the past as I could control startup action in Control Centre. It seems a recent update to Dropbox has changed the startup selection logic. I'm guessing but I suspect this is because of some developer work following the recent Ubuntu 18.04. Perhaps the developers don't know that their preferences tick box hasn't worked for a long time?