One Potion makes the bundle full on Bedrock , plus it cannot be mix with other potion since it's full* ,
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What I mean was , you can only put one potion in the bundle and it makes it full. Remember the bundle can handle 64 items/blocks in it.
Per the 1.21.30.23 changelog (emphasis mine):
A Bundle only fits one stack (usually 64 items) but it can be a mixture of many different item types
A stack of potions is one potion.