Iron golems hold the rose client-side 400 ticks too long. Server-side it already stopped holding the flower.
The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.
The reason for this is that the method net.minecraft.entity.monster.EntityIronGolem.setHoldingRose(boolean) sends the entity state 11 which sets the holdRoseTick field for the client to 400 even if the value is false. Instead it should send a different state id to reset the field for the client.