Really odd to see those things "out there"!
In real mode, the interrupt enable/disable status is controlled by the global CPU flag, in bit 9. If you saved the entire FLAGS register when interrupt was enabled with "pushf", and later you use "popf" to restore the flags, there's no reason to use "sti", since the interrupt bit was already restored, it was perfectly legal in real-mode DOS. However, in protected mode under an OS, it's unacceptable for a single program to disable interrupts of the entire CPU, so the global CPU interrupt flag is always enabled and controlled by the OS, but each program had its own virtual interrupt state controlled by "sti" and "cli", and cannot be changed by "pushf" and "popf" anymore.
What actually happened: This problem was already recognized in the beginning, documentation contains multiple warnings and examples of correct code, but developers ignored all the warnings and instructions, continued misusing "popf" and doing it wrong in as many different ways as possible.
I've also not written any asm in a couple decades...
so, thank you very much for explaining this to me. It makes total sense now!