When playing Geartown in Edison format, playing a 2nd field spell isn't letting it's eff go off?

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Hi. Replacing field spells used to count as destroying them until 2014 with the updated field spell rules. Under Edison rules, placing a set Necrovalley in my field slot already occupied by Geartown should allow it's activation and the special summoning of an ancient gear. Instead, no prompt comes up. The rest of the board was empty, and I had this happen in random Edison single matches on my turn 1. It's very easily repeatable.

Repeatable steps:

1. Place Geartown in a deck, make sure you have an appropriate target for its second effect.
2. Play a second field spell to replace it

Expected behavior:
Should get a trigger to special summon. Instead nothing happens.

[topaz wedge]

Did you destroy Geartown by activating a new field spell or by setting it ?

Geartown is a "when... you can" effect, so if you activated the new field spell, Geartown misses the timing.

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I set the new field spell