I think it would be more apt to issue a jury instruction that loss of evidence against law enforcement by law enforcement can be interpreted as a sign of guilt,

I think it would be more apt to issue a jury instruction that loss of evidence against law enforcement by law enforcement can be interpreted as a sign of guilt, similar to how pleading the 5th can be interpreted as a sign of guilt in civil cases. It does not conclusively mean they are guilty (accidents do happen), but it can be considered in cases that lack other evidence. Furthermore, if the person who "lost" the evidence can be found, they should be tried as a conspirator.