Have you tried manual partitioning? I found once I manually created /, swap and boot partitions like in the helpcenter docs, rebooted, it detected the new partition nicely.
https://getsol.us/articles/installation/disks/en/

For me workaround is using gparted and creating new partitions. Also you need to untick "boot" flag from windows EFI partition and mark newly created Fat32 partition flak as "boot". After that installer will see the new EFI partition.

Ran into this issue as well, instead of risking breaking dual booting between solus and manjaro, I popped my laptop open and removed my Manjaro drive while installing.

Hi,
I went into the same problem (I have around 10 disks !!). On each ESP partition (boot Fat 32 parttion), with Gparted (didn't work with Kdedisk manager (can't remember the exact name)), I unticked flag boot (it comes with esp), that changed then to msftdata with the manage flags menu.
If your disk is empty, then on install, it will ask you if you want to create an Esp partition on the disk you expect. I actually found this solution on the forum (can't find it again, but it's there).
I would say on your case, you untick the boot flag of the windows partition, and bob's your uncle. (by the way, you can install Gparted with the live iso, it works fine).

In T10324#201398, @Foid wrote:

Hi,
I went into the same problem (I have around 10 disks !!). On each ESP partition (boot Fat 32 parttion), with Gparted (didn't work with Kdedisk manager (can't remember the exact name)), I unticked flag boot (it comes with esp), that changed then to msftdata with the manage flags menu.
If your disk is empty, then on install, it will ask you if you want to create an Esp partition on the disk you expect. I actually found this solution on the forum (can't find it again, but it's there).
I would say on your case, you untick the boot flag of the windows partition, and bob's your uncle. (by the way, you can install Gparted with the live iso, it works fine).

Just to be clear, are you saying to remove the boot flag from a partition on the drive that you do want to install to?

What I am saying and have done is to select the Fat 32 partition, normally the ESP or EFI boot partition on the Windows disk (in this case), click on the right with the mouse, go to manage flags and untick the boot flag. It will untick the esp flag as well. Then, do the Solus install. Once it's done, you can reset the boot flag of the windows partition.
It will go back to boot and esp. It hasn't made any trouble on all my partition (Windows, OpenBsd, Mint, Solus and Fedora). It's just the time of the install. I found it here : https://discuss.getsol.us/d/4467-installer-not-seeing-new-efi-boot-partition