I had the same issue when choosing the manual installation path in the installer. The assigned swap partition was properly formatted, but does not get automatically mounted upon startup.
Inspired from the comment above I opened Disks (gnome-disk-utility), selected the swap partition in the Volumes visualization, then clicked the Settings gear icon, chose Edit mount options, disabled User Session Defaults. I did not change any settings as "Mount at system startup" was already selected before (but greyed out due to User Session Defaults being active).
Since then, when starting the system, it gets properly mounted automatically.
I'm running Solus 4.1 Fortitude, installed from USB, on a 12 inch MacBook Retina with 4 partitions on the SSD:
p1 (EFI fat32)
p2 (macOS apfs)
p3 (linux-swap)
p4 (Solus, ext4)
p3 was correctly selected as swap during installation, p4 as /