I'm sure Riot knows there are lots of bugs on Rek'Sai since they fixed quite a few of them through her lifetime. However, there are plenty of bugs left to fix. Sometimes your tunnel creates itself at right location but doesn't let you go through for some reason, attack speed on Q is somehow still soft capped with Hail of Blades unless you spam click your target to bypass the bug.
But the most annoying bug ever, for Rek'Sai players, is when you can die to attacks that happen the millisecond you ult not only killing you but making your ult go on cooldown.
It's an ultimate ability Riot. It's supposed to let you go untargetable instantly like Master Yi's Q, you don't directly go untargetable when you cast it which makes potential outplays and dodge with ult just impossible.
It's pretty easy to reproduce, try hitting Rek'Sai to kill her during her 0.35s cast time before she burrows (she does not instantly go untargetable like it said in the patch notes) and she will take damage and even die.