Tech research disappearing from tree

I noticed when I went into my tech tree, that there were several blank (unresearched) techs, most of which I know I already bought, including one of the first two you ever unlock. I'm finding myself having to purchase them again.

If this is a one-time bug, I'm fine with earning enough science to open them back up again, but I'm hoping there's a real fix. I have ships in orbit now with parts that are no longer available to me.

Some history -- have never enabled cheats of any kind, and I use no mods at all (and never have.)

I've already looked into "cheating" my way back to the tree I had before, but I'd rather fix the problem than band-aid the symptoms if at all possible. Anyone have any thoughts on what happened, and how to fix it?