If you haven't tried already, maybe try swapping the loader hose pairs at the quick-connects. So you're now using the boom circuit to operate the bucket cylinder, and bucket hydraulics to run the loader boom cylinders.
That way the problem will show to be a cylinder, not the hydraulic power circuit from the tractor being clogged or damaged.
Next I'd disconnect the bucket cylinders top and bottom lines so the cylinder can be manually pushed/pulled through its complete operating length without obstruction or noticeable defect, which will also force out all the existing fluid in the cylinder, that I'd drain into a clean pail to inspect for contamination.
It sounds like something got in the hydraulic fluid and is causing havoc somewhere in the system, that could be at the cylinder or anywhere in the lines all the way to the control valves.
Once drained, inspected, purged of old fluid, reassemble and start tractor then cycle the steering all the way both ways to purge main system hydraulics, and if all's ok then a full cycle of the bucket curl circuit in both directions will refill the cylinders and purge any air.
Hopefully it's all good!