I have encountered this issue, and it occurs when Playback Options > Timing > Note Durations > Default Notes (or other suboption) is set to 100%. This does indeed cause note-off events to occur at precisely the same MIDI tick as note-on events.
In some programs, such as Reaper, same-tick events are received by VST instruments with note-offs before note-ons, so they work correctly. In Dorico however, VST instruments do not appear to receive same-tick MIDI events in a deterministic order, so you must change the above setting to the next highest value, 99%.
If you export MIDI from Dorico with this set to 100%, and use it in Reaper, it works as expected.
But that might be because of the way Reaper handles MIDI events, so other programs might still have issues.
I previously brought up this issue here.
EDIT: I just realized you’re talking about chord symbols. I’m not sure if there’s a way to reduce the default duration of a chord symbol’s notes…
EDIT 2: It looks like if the above mentioned option is set below 100%, an extremely small duration is removed from the first of two successive chord notes with the same pitch, but it does not do this at 100%. It also looks like the chords respond to the Humanize option under Note Start Positions, so you may want to check that option, too.