According to the Minecraft wiki, suspicious stews should have an NBT tag that contains potion information and duration. However, it is not present in the beta and does not allow customization of suspicious stews. This might affect future editions, where the effect durations of suspicious stews obtained from trading/shipwrecks is random.
Comments 2
Thank you for your report!
However, this issue is Working as Intended.
In Bedrock, suspicious stew is implemented as an item with one of 9 data values, each of which determines both an effect and its duration. I don't know why this implementation was chosen, but it was clearly deliberate.
It's possible that they only implemented it this way because the the NBT method isn't currently compatible with the game logic and there is already a plan to change it in a future release. However, I'm only speculating about that: It could also be true that this is a parity break that they don't currently plan to fix. (Not every parity break is considered a bug.) In that case, you might want to suggest it on the Feedback site.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki
Still present in 1.13.0.15.