Elliott Plack wrote:

Has anyone else ever noticed this phenomenon, that mostly appears in
commercial structures? If you look at the water level in the toilet
bowl, it is constantly rising/falling about 1 cm. What causes this
exactly? Is it something to worry about?


It can be caused by wind pressure changes on the vent stack terminal.

Another common effect is from flushing large volumes of water
where gang toilet/urinal installations are present with flushometer
valves. The plumbing design takes this into account and vent
connections should be able to correct it leaving only small disturbances
in trap water levels.

In either case, Codes allow for trap disturbance of 1" water level or so.

No, it's not a problem .

Jim