We can make a change here so that the empty user row doesn't appear in the summary, but handling the per-content reaction cache is more complex, and just really not something that we attempt to do.
The "(deleted member)" bit will stay visible until such a time that enough people have reacted to the content that it doesn't display any more.
Rumour has it, that there may be add-ons that will rebuild the reaction summary for you if it is significant.