Posting this as this caused me 2 hours of headache trying to figure out what was happening.
I am developing a custom UI application with Forge and it was working well, I went away from my computer for 30 minutes, and when I came back, made some changes, and re-deployed, it was no longer working.
Turns out, earlier, I had tried to use ‘forge tunnel’ and it had not properly closed. I do not know what was happening as I couldn’t see any logging, but after I had deleted the container and runtime for the docker, it started working again.
To Atlassian, perhaps this is a bug that could be investigated or not. Unfortunately, I do not have any logs or ways to recreate it.
To Forge devs, hope this could help you if you find yourself in a similar situation.