Some time ago I encountered this bug and wrote about it in one of the channels. I haven't found the answer so I decided to write here.
Some time ago I played in the world created by me on a light mod build (Mostly there were visual or minor changes ie JEI NEA, etc., but stood quite famous mod From The Fog). I knew all the FTF lists perfectly well - what the mod changes and what it doesn't. Entering the world in the morning I noticed that all water blocks (sources) were replaced with blocks of purple wood. The entire river and nearby caves were dotted with these. When trying to break these blocks they would break at the same rate as the wood ( and would break faster with an axe ). When breaking a block it would stay in place, and when you pressed on it ( exactly after breaking it ) it would be replaced with the familiar water source. When trying to fall on a block I would get a fall death notification. At that time I had not built the portal to Nezer yet.
Upon re-entering the world, everything repaired itself in some unknown way. I am only writing in search of answers and trying to understand the reason for such an event. I only have these 2 screenshots from that moment. If anyone knows the reason or has had a similar experience, please give me an answer!

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