Hello Dorico team,
Sometimes when condensing is turned on, slurs within the condensed staves disappears. Unfortunately, the behaviour appears to be quite unpredictable, so I haven’t been able to pinpoint this to any specific cause. The existence of the slurs can be altered just by changing the number of staves in that system in any way possible within Dorico. I am not sure if this helps you in any way, but at least you are aware of it now (maybe you already were?).
All the best! 