Half of the partitions are empty on a KEY partition
You have 100 partitions. So it's likely that your hash values modulus 100 are all even numbers.
This is a general risk of hashing. If the hashing function produces only values with a pattern like this, you can get imbalanced partitions.
One way to fix this is to use a prime number for the number of partitions. For example 101 is a prime number close to your desired number of partitions.
This helps because if the hash values modulus 100 produced a pattern, that pattern will be offset by 1 per window of 100 values. Thus it will be spread more evenly over the partitions.