I have been yesterday in Home Center and seen there 18W T8 Osram Lumilux 865 (for a change, till now all triphosphor lamps there were warm white)

I put it in the lamp tester that is in the store (magnetic Switch Start), The lamp is standing vertically in the tester

The lamp powered up, with bright light in the ends and dim in the center. I assume this is the case due to it being brand new and low mercury

But besides it, the lamp also had a harsh 50Hz flicker, and its brightness through the flicker somewhat unstable. The gear in the lamp tester is ok, it did not do that when i tried it with a full mercury halophosphate T8 lamp

Is such harsh flicker normal for a low mercury lamp ? Will it stop after some initial period ? (how long ?)

i see this happen on new tubes, i bought 2X58w some weeks ago, and they tend to flicker when cold.

When a fluorescent lamp located in a cold area (Refrigerator or a freezer), it can have either a vibrating brightness (On magnetic ballasts) or standing waves (On electronic HF ballasts).
If the lamp start out very dim a the center but bright at the ends, so this is a brand new lamp, that wasn't used for a long time or that the mercury didn't released from the capsule/amalgam in the final stage of manufacturing.
Run the lamp continuously for several hours to get the mercury vapour pressure and distribution to reach equilibrium. Then this willn't happen again.