Currently even a single level of research removes the 300% limit on crafting them. Is it intended or a bug?

It is intended. The 300% limit for productivity is necessary for recipes that can be recycled, to prevent an infinite resource loop. Since science packs can't be recycled that's not an issue. And I think it also makes sense conceptually, because a science pack is kind of an abstract item.

Got it. Would it be possible to add productivity research for Corrundum's electrochemical science?

The mod should automatically add a productivity research for each science pack that's used in the lab productivity tech. If electrochemical science isn't included, maybe there's a Corundum mod setting to enable it? If it is there, it's possibly a Crucible bug.

Ah, that's it, it's not required by productivity tech research. Guess i should go and pester the Corrundum's author instead of you. Thanks for help.

BTW, the reason it's set up this way is because modded science packs aren't always usable in vanilla labs. Since lab productivity tech is pretty much the official "uses every science pack" tech, it's safe to assume that any mod that adds its science pack(s) to the tech also adds its science pack the the vanilla labs. And it's also a reasonably safe assumption that if a science pack is excluded from lab productivity, the science pack is probably used in a somewhat different way to vanilla science packs, and adding an infinite prod tech for it might go against that design.

Apparentely the electrochemical science just ain't used in productivity research, not a bug, it just isn't included for some reason. Unfortunately can't contact the author since he locked all discussions here and i don't have github account (moved to codeberg years ago). Oh well, maybe i just write a mod myself to add it :)