Brownout reset vs. SVSH(POR)

When supply power fails rapidly, the (programmable) reference for the SVS may drop before the SVS can trigger. So the SVSH comparator doesn't notice. However, in this case the slower BOR with its own (lower, fixed) reference will trigger and reset the MSP to power-on defaults.

If the SVS triggers a reset, this is a intermediate reset state. Not a complete BOR. However, the SVS holds the system in reset until low voltage condition ends, even if the current voltage would be sufficient for a normal power-on reset. (that is, if you reconfigured the SVS). After an SVS-triggered reset, not all hardware is reset, especially the SVS itself keeps its configuration. After a BOR, even the SVS is reset to defaults (of course, as without power, there will no safe configuration remain)

So your statement is somewhat incorrect: When you lower voltage so that teh SVS triggers, and then raise voltage, you do not power the system again, it was still powered, just held in reset. When you cut supply and a BOR comes, the system is powering-up.

Try this: you may program the SVS to trigger below, say, 2.2V. Now lower voltage. Around 2.2V the MSP will go into reset and when you rise it again, it will start with >2.2V and show you an SVS reset.
Now lower voltage to , say, 1.2V. When you rise it again, you wills see the MSp starting again at ~1.8V and give you BOR reset message. The SVS will be reset to deaults.

The users guide holds a nice diagram about how different reset states are building upon each other. Also, the register description shows you on which reset situation the registers are reset. Usually PUC (no brackets) but also sometimes curly? (POR) and round? (BOR) brackets.