Has anyone downloaded and used an App Inventor App on a phone that has Bitdefender (or some other antivirus software) running in the background? Does Bitdefender interfere with the app's ability to function?
In 8 years of App Inventor, only known commercial antivirus software (Avast and AVG) has had issues reported in this Community with App Inventor and those issues involve falsely detecting a virus in the App Inventor app.
See discussions.
All such software may have some effect on the ability of any app made with even the commercial compilers may have slight reduction in running execution speed. That is probably not noticeable for any practical purposes except when the software runs a full device scan. It may depend on whether the software continually monitors app execution or not (some virus software does that) other than during a scan . Bitdefender? Probably fine.
See this article
Pretty much depends on what the App does. If however any AV attempts to scan the memory the App is currently using, there is a risk of corruption. Also, most smartphones have AV pre-installed, it's not a good idea to have two AVs running at the same time - that may slow the system down or cause memory corruption. There could also be conflict between the two: on my PC I once had two AV programs to test their performance and each identified the other as harmful!
If you really think you need Bit Defender, try it. I'm not sure any AV is very effective, especially on Android. AV Heuristics tend to result in false-positives in my experience.