Same thing happened to me but I'm playing on my phone (Bedrock)
I have two sniffers that are nametagged, they were in a very deep enclosure I made that was filled with moss blocks, vines and glowberries. One of my sniffers could climb up the vines but didn't really bother to fix it, so I went on to do my business a bit further away from the enclosure. Few Minecraft days later, I walked pass my sniffer enclosure and immediately noticed that I only saw one of them in my peripheral vision. I immediately jumped in the enclosure to find only 1 sniffer left. I was confused, yet immediately thought one of them climbed a bit too high on the vines and fell onto their death, but it was strange because I didn't get a death message that it died.
Luckily enough I bred the two sniffers back then when they first grown into adults and gave the egg to my friend, so I have another spare. I borrowed the egg, hatched the sniffer and immediately nametagged it. I also secured the vines with string so it won't grow down in the enclosure for the sniffers to climb up. After knowing I finally secured all of the vines and glowberries, I immediately headed back to do my business elsewhere.
Another Minecraft days later and not being close to the enclosure, I checked the sniffer enclosure again to find the new sniffer I hatched gone, so I immediately suspected that something is wrong. I thought it just escaped but it was very unlikely because they were in a very deep enclosure, I secured all the vines and glowberries they can climb up AND sniffers are one of the slowest passive mobs in the game, so it was impossible for it to wander off so far.
I tried to search for the sniffer, just in case it somehow can escape. Also tried to find the old sniffer that first disappeared. Keep in mind, Im playing on a seed where you spawn in a cherry biome and the mountains are circular and a lake is in the middle. So my base, farms and all of the enclosures I made are in the middle of the circular mountain so it was really impossible for them to get lost or even go up the mountains because I know the area by heart and lit it up with lots of torches
I also tried to turn off all my resource packs, thinking it made the sniffers disappear but it didn't really work. I tried to relog over and over again but no avail. Though I noticed turning off all the resource packs isn't really the issue because the current and the oldest sniffer I have, that miraculously haven't disappeared yet, is working with the resource packs on and still thriving inside the enclosure.
Kuisongmei
Same thing happened to me but I'm playing on my phone (Bedrock)
I have two sniffers that are nametagged, they were in a very deep enclosure I made that was filled with moss blocks, vines and glowberries. One of my sniffers could climb up the vines but didn't really bother to fix it, so I went on to do my business a bit further away from the enclosure. Few Minecraft days later, I walked pass my sniffer enclosure and immediately noticed that I only saw one of them in my peripheral vision. I immediately jumped in the enclosure to find only 1 sniffer left. I was confused, yet immediately thought one of them climbed a bit too high on the vines and fell onto their death, but it was strange because I didn't get a death message that it died.
Luckily enough I bred the two sniffers back then when they first grown into adults and gave the egg to my friend, so I have another spare. I borrowed the egg, hatched the sniffer and immediately nametagged it. I also secured the vines with string so it won't grow down in the enclosure for the sniffers to climb up. After knowing I finally secured all of the vines and glowberries, I immediately headed back to do my business elsewhere.
Another Minecraft days later and not being close to the enclosure, I checked the sniffer enclosure again to find the new sniffer I hatched gone, so I immediately suspected that something is wrong. I thought it just escaped but it was very unlikely because they were in a very deep enclosure, I secured all the vines and glowberries they can climb up AND sniffers are one of the slowest passive mobs in the game, so it was impossible for it to wander off so far.
I tried to search for the sniffer, just in case it somehow can escape. Also tried to find the old sniffer that first disappeared. Keep in mind, Im playing on a seed where you spawn in a cherry biome and the mountains are circular and a lake is in the middle. So my base, farms and all of the enclosures I made are in the middle of the circular mountain so it was really impossible for them to get lost or even go up the mountains because I know the area by heart and lit it up with lots of torches
I also tried to turn off all my resource packs, thinking it made the sniffers disappear but it didn't really work. I tried to relog over and over again but no avail. Though I noticed turning off all the resource packs isn't really the issue because the current and the oldest sniffer I have, that miraculously haven't disappeared yet, is working with the resource packs on and still thriving inside the enclosure.