Once any base linked to the strat is captured, the strat/city retreats.
Once all bases linked to the strat are re-captured the city/strat will return to its forward location.
Definitely does not happen at all. This is an example from Compello today:
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Rooks have regained all the linked fields (and all other lost home bases), but the strats did not move back.
But the same thing goes for all large maps that have been up yet. On no map I saw the strat ever move back to their original location for any country. Once "retreated" they stay there.
With the usual flow of frontlines it means that after 1-3 days the strats of all countries will have become almost unreachable.