This is working as intended.
The sulphur cube will only absorb blocks in its allowlist, underwater TNT simply isnt one of those blocks.
This is because of a resource pack you are using, and is not a bug with Minecraft: Bedrock Edition.
Still present in Release v1.26.1 & Preview 1.26.10.25
This is working as intended.
The sulphur cube will only absorb blocks in its allowlist, underwater TNT simply isnt one of those blocks.