[BUG] Worker improvement pop-up ETA reports 1 for two turns [reproducible]

[ZXTT]

Bug involves workers' improvement ETA in the pop-up getting stuck at 1 for two turns.

Four saves attached showing progression.
70: two northern workers have ETA of 2 turns
71: two northern workers have ETA of 1 turn
72: two northern workers still have ETA of 1 turn
73: two northern workers are done

Played through a couple of times to confirm that it is reproducible.
Moved from 71 to 72:
Chose Iron Working for new tech.
Moved scout onto hill and healed.
Moved spearman into city
Range fired from city onto barb spearman to north.