Ok, so I noticed this before on a previous skill but thought maybe I was mistaken on the details so I didn't mention it then.
I sped up Penpersonship today from 9ish hours to 4.5ish hours. I switched to a different, slightly shorter skill to finish it up, then switched back. I learned for about an hour before I noticed I could speed it again. I was able to speed it from the just-under-4-hours time it had left to 1h45m.
Now, I'm not saying that I mind being occasionally able to do this, but I can tell it is not intended, so I figured I should let somebody know.