Designer had complained about this since at least OH 1.7. Frames within Frames has never really been supported. I suspect some of the phone apps have a problem rendering them.
It is just now that all the syntax warnings and errors are now being more strictly checked and enforced by OH.
In short, they have always been unsupported. You are now just seeing the warning in the log.
It is just a warning so you can continue to use them but realize at some point they may stop working.