When spiders are killed whilst pressing themselves against horizontal surfaces above them, their drops tend to glitch upwards through the blocks in a 'glass-elevator' style, sometimes ending up a long way away from the point of death. This does not happen in single-player or with a Mojang vanilla server.

I believe it is possible that the drops are being created a little too high on the mob's death, causing them to be partially inside the block above and glitching upwards. It does not show with other mobs but that may be because they do not press themselves upwards and their drops can still form in the (albeit small) space available.

To reproduce, create the 3x3x3 cube in the screenshot attached and spawn some cave spiders in the enclosure. Kill them and see the drops jump out of the top of the enclosure. Compare with vanilla mc where the drops just drop inside the enclosure.