Why does this simple select by primary key show such intensive CPU usage?
A possible explanation:

The table is nearly as big as innodb_buffer_pool_size.
The Could did some sort of maintenance just before that spike, hence the buffer_pool (cache) was lost.
UUIDs are terribly inefficient because of how random they are.
Each SELECT (during the spike) found the desired row to not be cached, so it had to read from disk.  (Was there also a spike in I/O?)

If the table is growing is size, the problem will become noticeably worse when the table becomes bigger than the buffer_pool.