Just don't talk negatively about your old job. "I'm looking to move because I want to have more impact". "The work I'm doing now is not core to company's success and I want to have more impact". Etc.
I never looked down on someone who was doing boring work if that is what they were supposed to be doing. I'm looking for people who can communicate well and have strong core fundamentals. If you can demonstrate those things then you're doing well.
What kind of red flags are you worried about?
This would tell me that you're really engaged, up to date on the latest methods, and motivated to make things around you better.
For example, if you couldn't remove a bad pattern because it was too deeply entrenched for you to handle it yourself, and you couldn't get anyone more senior interested: that's expected for a junior developer and probably wouldn't be held against you.
What types of things do you say and why do you feel they may be red flags?