Correct. I do not maintain backwards compatibility when publishing UI3 updates. Breaking changes happen fairly often. Sometimes the changes are minor and you probably wouldn't even notice. But in recent months the changes have been much more significant, hence the problem you've been having.
In fact it isn't always entirely safe to update UI3 manually even if you are on the latest
Blue Iris version. Sometimes I get advanced notice of a coming Blue Iris change such that I can prepare a UI3 update to go with it. UI3-132 was one of those updates, which broke one small thing so that a related very big thing wouldn't break when 5.3.6.0 was released. In this particular example, the Blue Iris developer decided to make his change a different way without breaking existing apps, such that the UI3-132 release was no longer needed (which is why BI 5.3.6.0 still comes with UI3-131) . It is all pretty complicated. Suffice it to say, the safest thing to do is not update UI3 manually unless you're on the latest BI version already and the UI3 update has a bugfix you need and you can't wait for the next BI release to get it.