This is normal, and by design. 
Essentially what happens is that FSRS assumes that, if after such a long delay, you still remember that material that you must remember it quite well. And so it pushes the intervals off into the distance
There isn’t much to do to ‘fix’ this, as it’s how the algorithm is designed to work and everything is operating as intended.
Your options are roughly three-fold:
- Let FSRS do its thing as normal.
- Use filtered decks to sidestep this behavior.
- Reset the deck.
Please let me know if that answers your question, and if there’s anything else I can help with.