Grass and mycelium do not decay when under a flowing water block, this flowing waterblock can not take up a full block. In older versions of minecraft every waterblock (taking up less than a full block) would also result in grass and mycelium decaying.
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I've tried to search for it but I guess my way of describing the problem didn't coop with his bug report.
Hi there!
This looks like a duplicate of MC-192664. Make sure to do a quick search for your bug before posting it, and if you find it, vote for it, comment on it, and follow it. This is the best way for it to be prioritized by Mojang. Making a duplicate post means more work for the volunteers who help out to moderate and clean things up for the developers to look at and fix what needs to be fixed.