I found a temporary fix for the disappearing senator bug that has been submitted to bug reports. I hope this is the right place to put such a thing, but I do know that I'm not the only person who has experienced this problem so I wish to help in any way I can.

To fix your senators:

1. Save your file.

2. Use dev mode to regenerate senators. If you hadn't gained any favor you can stop here.

3. Save your file again UNDER A DIFFERENT NAME.

4. Navigate to your save files on Windows you can do this by going to C:\Users\[INSERT YOUR USER HERE]\AppData\LocalLow\Ludeon Studios\RimWorld by Ludeon Studios\Saves
The wiki has tutorials on how to access your saves on other operating systems.

5. Open both of the saves you just made using notepad or a similar word processing program.

6. In your first save hit ctrl + f and search for "<li Class="VFEC.Senators.WorldComponent_Senators">" going down the file. This is where the game stores the references for which pawns are your senators and which ones you have completed.

7. Navigate to any senators who it says you have gained the favor of in this menu. There will be 3 factions in this list 1 for each of the republics. Any senators you gained favor with will have the line <favored>True</favored>. If you don't see this line you can ignore those senators.

8. Copy all of the data for one of your senators who you have gained favor with.

Example Data:

<pawn>Thing_Human12362149</pawn>
<quest>Quest_56</quest>
<favored>True</favored>
<canBribe>True</canBribe>

9. Open the second save file you made and repeat Step 6.

10. Find the location where your senator was in the list i.e if it was the second senator of faction_40 then go to the second senator of faction_40 in the new file.

11. Replace the senator data with the senator data you copied. If in the correct slot this should effectively move your senator to the new save file where all the regenerated senators are. Repeat steps 8, 10 and 11 for all desired senators.

12. Load the new save and delete the old and rename it to whatever you wish. You should now have all the empty null senator slots filled while maintaining your progress in uniting the republics.
If there are any issues with this let me know.

Mod developers thank you for the mod I apologize if this is the wrong place for my guide on how to negate this bug, but once again ty.