I would lead with questions for both context and for you to determine your plan of action.

Its “always been” squishy…how long have you lived here? 1 year? 10 years? It matters in the context of if it was a quick flip install to sell the place and you’ve only been living there a short time, the rest will probably be falling apart soon too. If its been a long time, it could point to just that spot being an issue and needing repair.

There’s also the question of your time and budget. There’s a good chance the entire floor needs to be replaced, but if that’s just out of the question then a patch job could keep it together a very long time if done right.

What room is that? Bathroom? Mud room? There’s tile on the wall too so I’m wondering if water is playing into the problem (either in the tile install or damage to the sub floor in that spot).