Dealing with possible unprofessional behavior and retaliation from former-manager

Question

I have transferred to another team, and my former-manager is raw about it. He asked me to help with a task before my transfer, and I agreed to help even though I wasn't obligated to help with it.

I worked on what I thought was going to be a simple task, but it turned into something more complex. Since I really wanted to step away from the responsibilities of my prior role, and focus more on working on things for my current role, I thought it was best to let a more senior co-worker take over that task. I didn't want to be unfair to my current manager, and keep working on a task that I thought was turning into possibly a longer one, time-wise.

The senior co-worker was able to finish the task in a couple of hours.

My former-manager then sends an email to us both, thanking the senior dev, but emphasized the fact that I wasn't able to finish this on my own. He also referenced language from my current team, I think in an effort to highlight that I don't belong there?

A couple of things about this:

1) I don't even have official access to the platform the task is reliant on, and I have not been using it directly during my work.

2) While I admire the senior dev, and want to be more like him, I think it's unfair to point out that I wasn't able to do this on my own- the senior dev has 20 yrs of industry experience, I have 5.

3)While I am more than willing to learn that platform, the company doesn't give access to everyone. I don't understand how I was expected to divine an answer to this without the right tools or experience.

Questions

1) Was it a good idea to involve the senior dev, or should I have kept working on it. The solution he came up with was something I found myself, but as mentioned I wanted to move on from this, and so I didn't focus enough time on that.

2) How do I keep former-manager from bad-mouthing me to current one?

3) How do I calm down former manger's raw feelings about me leaving?

Answer 1

Be professional, even if the former manager isn't.

Simply respond, with a "cc" to that developer something along the lines of -

Thanks, Mr. Former Manager, for the helpful feedback. Hopefully I'll be able to apply the lessons learned from this to my new position. I'd also like to thank Mr. Senior Developer for stepping in when I realized that the scope of this task was much greater than initially presented, and when I realized I did not have the access to the tools and resources to fully complete this task in my remaining time with the team. In retrospect, it was probably a mistake on my part to have agreed to take on this request. I wish you both, and the rest of my former team, the very best success going forward. Thank you for the experience of working with and learning from you!

And keep it at that. If you have a regular sit-down with your new manager, you might want to mention, asking to keep it low key, that you are worried that the former manager has some hard feelings over you leaving the team, as an FYI.