Kelp grows in flowing water and creates water source blocks. The new source block spreads and creates new flowing water. By that a chain reaction happens which can create an infinite amount of source blocks

My realms world just updated and this completely destroyed my kelp farm which relyed on the mechanic that kelp would break off when it grew beyond the source block it was planted in and into the flowing block above it. (see Skippy6's automatic kelp farm on youtube for the mechanic)

This was an issue that was seen on the Java side, but wasn't opened for MCPE