hmm i dont think i ever came across a bug like that. but... usually i made serums that were way stronger and effective than the blue purple red anyways xD
edit: i dont think i ever used "check stock". i only used the production stockpile
edit2: yeah just checked my game. dont see any check stock or whatnot. only production stockpile and it's never had anything weird.
In the early part of the game you can't really create the best serums because you haven't unlocked the higher tiers of research, so recreating the Blue, Purple and Red serums is an easy way to get some basic serums for use early on. I have a number of custom serums that I create later on, once I've unlocked and researched all tiers, especially the top tiers you obtain from Nora. With the right combination of traits obtained from Nora you can create some of the best possible serums! Well, I'm guessing those are still as effective as they were 3 years ago, I've not yet gotten far enough into the game to actually see how they perform nowadays.
"Check Stock" is the button at the bottom of the company stats over on the left side of the screen. Top part is the player related stuff: Outfit Manager; Check Inventory; Character Sheet; Perk Sheet; plus the player stats. Middle section is the Goal Information. Bottom section is the company stuff: company name; Employee Count; Company Funds; etc. The very bottom button is "Check Stock". When you click on it you get a new window that shows the total serums you have waiting to be sold or sitting in your production stockpile, basically it's your total business stock. That button can be clicked any time, anywhere, you don't need to be in any of your company areas.