Unable to add a GPT partition with 400+GB free space

[TJ]

20.04.1 installer, Custom storage layout

On a device with 400+ GB free space and 4 existing GPT partitions the option to add a GPT partition is disabled.

Apparently the logic cannot code with dis-contiguous partitions!

root@ubuntu-server:/# gdisk -l /dev/sda
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 1953459632 sectors, 931.5 GiB
Model: LOGICAL VOLUME
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 05564500-64D5-40BE-A6EE-39476C575604
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 1953459598
Partitions will be aligned on 2048-sector boundaries
Total free space is 843898221 sectors (402.4 GiB)

Number Start (sector) End (sector) Size Code Name
   1 2048 206847 100.0 MiB EF00 EFI system partition
   2 206848 468991 128.0 MiB 0C01 Microsoft reserved ...
   3 468992 133304319 63.3 GiB 0700 Basic data partition
   4 977199104 1953458175 465.5 GiB 0700 Basic data partition