Road wet
Never never ride on the white line and white arrows, and brake, especially when your bike do not have ABS.

Even me riding over the white arrow on easy constant throttle, on a road inclining up ... At times can feel my rear wheel spin/slip a bit.

The risk you took was not because your are riding a bike

You took risk by riding on the white line on a wet road.

Not only white lines or the white arrows are slippery.

This flyover/overhead bridge expansion joints, which are made of steel. This is super slippery when it's wet. You can feel your tyres moving side ways especially whenn you are cornering over it. I have seen many riders and even cars lost grip and crash over it.

I remember this incident happened in jln semarak in the morning.
Was approaching a traffic light, and noticed a scooter following closely behind me. The light turned amber and I stepped on the brakes to slow down preparing to stop.
This mf was tailgating me panic Western he saw my brake lights. He must be thinking I will beat the red light.
He panic locked his wheels, no abs and crashed onto the road behind me. Saw the whole incident on my rear mirrors.

Lol.

Thankfully I didn't get beaten up. The biker admitted it was his mistake

I know marital arts self defence.
No scared

That's because I don't know
Lol

That metal expansion when wet round, especially when you are cornering over it , don't make sudden acceleration. Keep it stay.
I usually just roll over it. Even on my. Michelin road 5 can feel the movement.