It seems that no one has brought it here, so:
On the official .fr forum there is
the following announcement
Bug: drop rate, xp and tp bonus
There were some tests on the effectiveness of bonuses to drop rate, xp and tp performed internally by the Runes of Magic team.
The results highlighted a bug:
If your bonus is 500% yours actual bonus drops to 100%.
To make it clear, if your cumulative rates give you 550%, in reality, you will only enjoy a 150% bonus.
If you intend to farm, do not exceed 499% in your interest until an eventual bugfix.
It matches what
was observed earlier, but some ppl have tried and
cannot reproduce this. So it might be bound to how some specific bonus (pot/title etc.) works, and not be a general 500% border value.
Also what is interesting is that previous reports were focusing on drop-rate only, while this official French statement mentions all three: drop-rate, xp and tp buffs.