You'll get several advance warnings if your SOC gets low prior to the system starting to shut down functions to try to preserve power. The time you'll notice you can't keep charging up a long grade will be as the SOC gets into the low single digits. On the flat, the vehicle can hold charge up to about 75-mph, but the REx can't keep up if you throw in a grade, you've got the a/c or heat roaring, maybe the lights fully on, etc. The REx will only try to bring the charge up to either 6%, or the point where you manually activated it (requires coding in the USA). It will not try to raise the charge about 6% unless it was already higher, and you manually turned it on. WHen you shut the vehicle off, the SOC that it currently holds becomes the new maximum the REx will try to hold, so it won't recover more than to 6% on its own.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)