You could try the community patch, as it fixes many of these bugs.

https://www.games2gether.com/amplitude-studios/endless-legend/forums/15-modding/threads/30618-endless-legend-community-patch

Issue: I am also having this issue(Urkan vanish at end of turn.) I did look at the community patch but I personally don't really like the balance changes. Please officially address this. 

I had two Urkan simply vanish and I had no idea why. I thought it was maybe a natural behavior as both of their health was low, but realized it was likely not when they were gone from both cities and armies tabs. So I tried reloading only to have it crash(worldgen, not game crash) with an error: "Game creation has failed (Argument cannot be null. Parameter name:gameEntity)." File and autosaves are now completely screwed. Urkans vanished after I had uprooted them and I was moving them to attack the one faction I was not peaceful with, one at a time, one several turns prior to the other. I was playing the Mykara faction, around 260 turns in. I had a fair number of custom world settings. No mods. Playing on Steam. All DLC enabled. Single player. I have tried verifying game integrity.

• Specs - 

Resolution: 1920x1080

OS Name    Microsoft Windows 10 Home

Version    10.0.19041 Build 19041

Other OS Description     Not Available

OS Manufacturer    Microsoft Corporation

System Name    GILGAMESH

System Manufacturer    To be filled by O.E.M.

System Model    To be filled by O.E.M.

System Type    x64-based PC

System SKU    SKU

Processor    AMD FX(tm)-9590 Eight-Core Processor, 4700 Mhz, 4 Core(s), 8 Logical Processor(s)

BIOS Version/Date    American Megatrends Inc. 2501, 4/7/2014

SMBIOS Version    2.7

Embedded Controller Version    255.255

BIOS Mode    UEFI

BaseBoard Manufacturer    ASUSTeK COMPUTER INC.

BaseBoard Product    M5A99FX PRO R2.0

BaseBoard Version    Rev 1.xx

Platform Role    Desktop

Secure Boot State    On

PCR7 Configuration    Binding Not Possible

Windows Directory    C:\WINDOWS

System Directory    C:\WINDOWS\system32

Boot Device    \Device\HarddiskVolume2

Hardware Abstraction Layer    Version = "10.0.19041.1"

Installed Physical Memory (RAM)    16.0 GB

Total Physical Memory    15.9 GB

Available Physical Memory    9.32 GB

Total Virtual Memory    18.3 GB

Available Virtual Memory    9.22 GB

Page File Space    2.38 GB

Page File    C:\pagefile.sys

Hyper-V - VM Monitor Mode Extensions    Yes

Hyper-V - Second Level Address Translation Extensions    Yes

Hyper-V - Virtualization Enabled in Firmware    Yes

Hyper-V - Data Execution Protection    Yes

1. Version -

1.8.2 S3 (64-BIT)

1. Game save - You can find it here:
   
   Mykara - Turn 264.sav

2. Output_log.txt:
   
   output_log.txt

3. Diagnostics.html -

   • Diagnostics.zip

   • There were 4 of these, and I had no idea which one you needed, so I zipped and sent all 4.

4. Registry.xml - [Documentsdir]/Endless Legend/Users/[SteamID]/Registry.xml

        Screenshots - Screenshots are pretty useless here, and I had not taken any, sorry.