ASA NAT to multiple outside IP addresses

We are hitting port exhaustion on our single-address NAT. Trying to change the NAT object to a range results in loss of connectivity.

It works when I run it as:

nat (Inside,Outside) after-auto source dynamic INSIDE_SUBNETS interface 

But not when I run it as

nat (Inside,Outside) after-auto source dynamic INSIDE_SUBNETS OUTSIDE_NAT_POOL 

Pool config:

 sh run obj net in | incl NAT_POOL object network OUTSIDE_NAT_POOL range 1.2.3.4 1.2.3.5 

Where am I going wrong? There are several other after-auto NATs that are working correctly, but are all configured with a single IP or configured to use the interface.