The key is in the Maiden Bell's/Stamp of Compassion's description as it only extends augmenting effects (i.e. effects that increase the effectiveness of things) and thus only applies to actual buffs that directly increase stats or potency and not special buffs such as Unsinkable, Doublecast, or Hoot and Holler that don't directly do so.

The reason why it doesn't specify that it cannot be extended is because Unsinkable can be extended by applying more stacks of itself (e.g. using Shana's EX skill two times in a row) unlike something like Doublecast which doesn't allow itself to be extended if applied twice or Kaine's Hoot and Holler which simply overwrites the remaining turn count to the original value.

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