I had in mind Unconditional Surrender and Star Empires (I don't remember if they have solo campaigns specifically, but they can be very long). But it looks like they have the same issue, at least I could find a table with the same symptoms (duration reported as 35 days, but it actually lasted 40-41 days): https://boardgamearena.com/table?table=362428244
And this is probably why it rang a bell: https://boardgamearena.com/forum/viewtopic.php?p=205814
It seems the developer added a save/load option as a workaround (or more likely: that as a side effect can be used as workaround).
And this is probably why it rang a bell: https://boardgamearena.com/forum/viewtopic.php?p=205814
It seems the developer added a save/load option as a workaround (or more likely: that as a side effect can be used as workaround).