I figured it out. The story is quite convoluted (and I'll spare you all the details), but the RXP gauge is working correctly, so you can mark this as solved (somewhat). It all stems from LM not relying on official government information for airport navigation data, but rather what's posted online in those flightsim navigation websites, like SkyVector, etc.. LM's stock airport in P3d5 has the tower frequency as 125.400 and this value has translated across every add-on, including anything that use
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