Thanks for the kind explanation.

From the 80C51 family hardware description from Philips:

The polling cycle is repeated with each machine cycle, and the
values polled are the values that were present at S5P2 of the
previous machine cycle. Note that if an interrupt flag is active but not
being responded to for one of the above conditions, if the flag is not
still active when the blocking condition is removed, the denied
interrupt will not be serviced. In other words, the fact that the
interrupt flag was once active but not serviced is not remembered.
Every polling cycle is new.

Than, if I do not clear interrupt flags with sw, the only way the above condition to occur is the fast changing level-activated external interrupts.

Right?