I spent some more time on this "challenge" and believe I came up with a working configuration!
Found a bullet in one sample configuration saying "If packets that match a policy node do not need to be forwarded according to PBR, specify deny match mode for the policy node". I replicated this in my configuraton and noticed that the switch started to treat my traffic the way I wanted.
I have configured two ACLs, one matching traffic from subnet A to local (CorpNet) subnets and one matching traffic from Subnet A to any destination (Internet). I then built one PBR with two nodes based on the two ACLs, the first node is set to deny match mode. The other is set to permit and applies an alternative next-hop address (my new ISP). Lastly, applied the PBR to the VLAN interface of Subnet A.
/f
acl number 3005
rule 5 permit tcp source {Subnet A} 0.0.0.255 destination {CorpNet}
acl number 3006
rule 5 permit tcp source {Subnet A} 0.0.0.255
policy-based-route PBR1 deny node 1
if-match acl 3005
apply ip-address next-hop {IP-address of A5820X}
quit
policy-based-route PBR1 permit node 5
if-match acl 3006
apply ip-address next-hop [NewISP_GW}
quit