most likely you have a T&G joint which is not over a joist. But chpboard is quite weak and it might just be a section that's given way. Especially happens in front of sinks and at the bottom of stairs.
If it was me (and I could be bothered) I'd use that as an excuse to take up all the flooring in that room and replace with 18mm ply.
do your best to have all joints over a joist, but nog them otherwise.
If you have to cut it to have any narrow strips, try to put them over plumbing or cables that might one day need attention. Pencil the boards to show cables and pipes below.
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