My toilet flushes normally, but after the flush, the water drains out again slowly until the siphon suction breaks (this happens about 10-15 seconds after the flush). This doesn't always happen -- I'd say 40% of the time.
- Turn off water to toilet.
- Flush it.
- Empty rest of water in tank. Often times a towel is easiest way.
- When it is really dry remove the flapper.
- Clean bottom of tank by the hole, sand it (any sand paper). Just make the hole look new, no need to overdo the sanding.
- Put flapper back on.
- Turn on water and try it out.
If that does not work order a new flapper and install that. Since you have cleaned the tank the new flapper will work better and last longer. The extra calcium/gunk that forms around the hole makes flapper lifespan shorter.
I have had this happen on toilet models that have a vertical tube and not a flapper. These are called no-flapper style. Here is a link to a particular model: https://www.plumbingsupply.com/mansfieldtoiletparts-claro.html
The vertical tube looks sort of like an upside down trumpet, where it forms a seal at the base of the tank. Two things can happen with these. One is leaking at the bottom due to calcium buildup, that can be cleaned with vinegar and a toothbrush. There is also a gasket that might need to be replaced. The other is that the flush valve gets stuck on the part of the tube where the flush valve inserts. Cleaning off the flush valve arm to make sure it is smooth and slippery, and doing the same to the part where the flush valve arm inserts, has helped. But ultimately, what I ended up doing with these in the long run was to take the toilet apart and replace them with the flapper style.