With great power comes great responsibility, and it looks like that's been abused a bit. Unlike the old-formats where it'd quickly lead to huge mess, nothing in new-formats really prevents people adding to other people's styles -
but you still shouldn't do it.
It's understandable that you might want to create extra pieces that go with an official style or someone else's style. However, if doing so, please put these pieces in your own separate style. NeoLemmix allows mixing tilesets in a single level. Yes, it can be a bit annoying to switch tilesets (maybe the editor could use a feature to display multiple sets at once in the selection area? - but you'd have to convince Nepster that's worthwhile), but by adding them to someone else's set, you're creating the impression that they're responsible for that piece - and indeed, they might feel compelled to maintain it in the future, so their style doesn't look like it has a broken piece. In the case of official styles, this is essentially pushing that same thing on the maintainers of NeoLemmix itself. Some creators might also feel the quality isn't up to standard (or conversely, that it's too high, if they're going for a deliberate lower-quality aesthetic), or that the new piece doesn't have the right feel they were going for with the set, etc - and it's being pushed into their style anyway.
If you're creating pieces for use in a single level, consider having a "(your name)_special" style, and include the relevant pieces with the pack the level appears in. For examples of this, see Lemmings Plus III, IV or VI.
From now on, all additions / modifications to graphic sets must be submitted by the graphic set's author (or one of them, in the case of a set with multiple authors). For the purposes of this rule, NeoLemmix's creators are considered to be the authors of the
official graphic sets and any abandoned graphic sets - we will, of course, take community discussions into account when deciding what we will or won't accept, but we have the final say; and of course, you are welcome to submit your proposed changes to a graphic set's author, but it's their call whether it gets included or not and they must be the one to submit it.
We'll be reasonable here - eg. if Alice decides that she's no longer going to maintain her graphic set and agrees for Bob to take over maintaining it, we won't require Bob to send all changes via Alice anyway; Bob can submit them himself. But Bob can't randomly decide to add to a set that Alice is still maintaining, unless that set is a collaborative effort between both of them in the first place. Likewise, in the case of abandoned sets, we're happy to let known forum members take over maintaining them - as long as this is a genuine taking over of the set, and not just claiming such short-term to slip in one or two new pieces.
My initial intention here was to preserve the already-added ones, however, many of these go as far as being deceptive, and should not be left in the official styles in any capacity. As such, I'll be deciding on a per-piece basis.