The source code (BsonClassMapSerializer.cs) appears to have the following statement around .BeginInit and .EngInit calls:
#if NET45
...
#endif

The #if statement was added to avoid dependency on unsupported API where it's not available, but in my case, where NETCOREAPP2_0 is defined instead of NET45, casting document to ISupportInitialize then calling InitBeginand InitEnd seems to work perfectly fine.

Can we change the #if condition to something that captures more version that support the feature, or is there another reason to keep it? (If so - would be nice to document it)

This created a lot of confusion for me since the docs don't specify version requirement for this feature to work, so I couldn't figure out why EndInit wasn't being called.