The long mundane potion is great for brewing extended weakness potions. however, an unfortunate glitch makes it unobtainable in survival. My guess is that an update to brewing, possibly the update that added phantoms, update aquatic, 1.16 or 1.21, caused the recipes of some potions to change ,specifically the mundane potion as the phantom update added phantom membranes to the recipe and 1.21 added breeze rods, cobwebs, slime blocks and stone to the recipe and maybe while adding them a developer saw that redstone, which was used by long mundane potions, wasn't on the list, as it is on java where there is no long mundane potion,and added it to the list. This may have caused the mundane potion to take priority over the long mundane potion and thus making it impossible to make.
Steps to reproduce:
1.get a brewing stand
2.place blaze powder in the fuel slot
3.add 1-3 water bottles in the bottom
4.add redstone dust into the ingredient slot
This will cause the potion to start brewing as expected but when you take it out it will be a regular mundane potion. Even if you try to add another redstone dust it will not brew
Expectation:1-3 long mundane potions should be produced
Reality:regular mundane potions are created
How mojang/Microsoft can fix it is by removing redstone dust from the recipe for the mundane potion and adding it to the long mundane potion. This will not make mundane unobtainable as you can still use ingredients like magma cream to make it and has the benefit of making long mundane obtainable
Thank you for reading to the end and please fix this as, weirdly, the long mundane potion is my favourite way to brew extended weakness potions.
For anyone wondering, you can make a weakness extended by adding a fermented spider eye to a long mundane potion. Please vote for this bug to be fixed