To properly use the builtin upgrade system you need to define both. A group and the next upgrade.
The problem: I modify the vanilla combinator and most mods use deepcopy() to make their own version of an prototype. They also copy my changes and don't reset them.
This usually applies to mods that were available in Factorio v16 when next_upgrade wasn't available.
I will move changes to the vanilla constant combinator to data-final-fixes in the next update. That should get around older mods which don't reset next_upgrade when copying a constant combinator.