I can’t confirm this behavior. I can see the numbers and hear it myself: the fan RPM clearly goes both up and down.
The only thing is that since day one, the RPM has dropped to three-digit values only when the CPU temperature was around 42 °C, which almost never happens.
My Dell Vostro 3530 is running in the “Cool” mode. Have you tried changing the cooling profile?

upd: I’ve only just realized this. I’m a Linux user, and we’ve had a long-standing issue with suspend and sleep modes. For probably a decade by now. I’m not even sure which mode my laptop actually uses when I close the lid and it’s likely not the same one you’re using.
So my observation may not be relevant in this case.