Whether I place x1 underground belt using CTRL+B or x50, it takes around 5 to 6 seconds and sometimes when I hit 'R' on a connector, I lose all the items that was being transferred..?
I was transferring plastic (around 100) and hit 'R' on one end of the underground connector and they never reached the end of the underground belt...

Yes, beltlayer only transfers items from above ground to underground or vice versa once every 5 seconds to save you UPS. You should not lose any items when reversing the direction, although it may again take up to 5 seconds for items to start flowing in the opposite direction.

Understood !
This may also be why items on belts loses their side. If I have Copper and Iron on one belt (half-half), they get combined when going underground which sometimes causes issues. Just my two cents :)

From the description:

Items are mixed randomly in the left and right lanes of output belts. This makes Beltlayer only usable with belts carrying the same item in both lanes, or "sushi belt" designs that don't have any item placement requirements. This is a limitation of Factorio's loader entities.