If it crashes at 800k polys, there is probably something wrong with your system or there is something more than just multires involved.
Most of people are , at least, able to reach 3 millions and some can reach one, two or three dozens of millions of polys.
That being said, it has been frequent during 2.8 and 2.9 series, to see newly introduced features, not immediately supported by multires modifier.
Ace_Dragon already said it. Other developers are involved.
So, there is a latency about when new stuff is supported in multires.
Just saving .blend file after addition of modifier should solve several of annoyances.
Most of times, users have been warned in release notes that a feature is not supported by multires modifier.
But it is always possible to discover an unknown bug.