innodb_buffer_pool_size more data than pool size
If the buffer_pool is set at 26GB in a 64GB server, what is the rest of RAM being used for?  Normally you should simply set it to 45-50G and not worry.

On the what-if question:

Case 1:  There is high "locality of reference".  A buffer_pool that is much smaller than the total data (24.3G in your case) works fine.  The buffer_pool is a "cache", and this Case is an example where caching is important.

Case 2:  Lots of random access or lots of table scans.  In this Case performance will degrade, possibly seriously, if the buffer_pool is not big enough for the dataset.  The caching fails, and I/O ensues.

UUID primary keys or other indexes can lead to random accesses and performance problems in Case 2.

I see no clues in what you said that could explain "32gb .. caused stability problem".

In most servers, O_DIRECT is good.

innodb_buffer_pool_instances should probably be 16 in your range.