Similar problem to my earlier report but not sure if it is to do with unit cycling or diplomacy interaction. 

saved files and diags follow.

Diagnostics (2020'12'23 @0825'14''849''').html

Diagnostics (2020'12'23 @0825'14''849''')p2.html

Diagnostics (2020'12'23 @0825'14''849''')p4.html

Diagnostics (2020'12'23 @0825'14''849''')p3.html

auto-save 592.ctr

auto-save 591.ctr