Glad to hear it worked! Please share the kudos and mark the answer as accepted 🙂
Can't help you too much with the inconsistency. I haven't had any issues like that. It's great that the legend has a lot of options so we can make the best legends... but it also makes it tricky when you just want it to do its thing :). If you continue to encounter it I'd recommend trying to reproduce it cleanly and then logging a bug with support. i.e. if you have a map with a layer, create a layout with legend + new layer sync enabled, then add a new layer and it doesn't appear in legend - that could be a support case as it's not expected behavior. Without the reproducibility it's very difficult to know what happened given the range of workflows and configurations.
Either way - glad it seems to be working OK now!