I noticed that too especially in kunlun, when event is on if u stack too much drop rate good items drop less and shitty items drops alot more, people say common items override your chance to get good/rare drops so you're better off without the drop deco but gm says otherwise, of course gms are not grinding so im not sure what really goes on. Do a test with and without drop deco and see whats best, this weekend is x2 but i tested on x3 already and if u use deco or drop card is a mess and just waste of time ( excluding CCD runs). I wonder if it will ever get fixed since GMs says is 'normal' and increase drop rate don't affect chances of good stuff but reality is very different.
Don’t use any drop deco during events unless you’re doing dungeons like GCM and lower...GMs don’t actually play game,they just tell you what they do on a test server. If you going to grind in KL during events try not to have over 30-40% drop..if you got just inner pants lvl2 that should be sufficient. The only time to ever go over 50% drop in KL is when you farm in only one spot like hounds/rats/etc