Transmit power limited to 14 dBm on TL-WR1043NDv2/v3

Greetings! This is my first post here, since so far I've been able to more or less solve any issue I had with openwrt with just searching the wiki/forums.

I currently have two routers running custom images of OpenWrt 19.07.7. They're far enough away to not interfere with each other and there is generally no other interference present since this setup is in a very rural part of the country.

A few days ago I noticed that the wifi coverage is lower than it used to be. I checked the settings and this is what I saw:

1043v3

14 dBm (25 mV) seems pretty low, even for my usecase (I've had both routers running at just 16 and 18 dBm respectively before just fine).
I've read https://openwrt.org/faq/other_transmit_power_issues -- but even this page only mentions "The reg domain allow 23 dBm (correctly set) while I'm unable to go above 16 dBm" as a common issue, but 14 dBm seems far too low to be normal.

I checked iw reg get. For some reason two country codes are listed but the "global" one seems to be applied at the UI, since the max transmit power is correctly listed at 20 dBm.

root@OpenWrt-TL1:~#  iw reg get
global
country HU: DFS-ETSI
        (2400 - 2483 @ 40), (N/A, 20), (N/A)
        (5150 - 5250 @ 80), (N/A, 23), (N/A), NO-OUTDOOR, AUTO-BW
        (5250 - 5350 @ 80), (N/A, 20), (0 ms), NO-OUTDOOR, DFS, AUTO-BW
        (5470 - 5725 @ 160), (N/A, 26), (0 ms), DFS
        (5725 - 5875 @ 80), (N/A, 13), (N/A)
        (57000 - 66000 @ 2160), (N/A, 40), (N/A)

phy#0
country US: DFS-FCC
        (2400 - 2472 @ 40), (N/A, 30), (N/A)
        (5150 - 5250 @ 80), (N/A, 23), (N/A), AUTO-BW
        (5250 - 5350 @ 80), (N/A, 23), (0 ms), DFS, AUTO-BW
        (5470 - 5730 @ 160), (N/A, 23), (0 ms), DFS
        (5730 - 5850 @ 80), (N/A, 30), (N/A)
        (57240 - 71000 @ 2160), (N/A, 40), (N/A)

I did some tests with my older router which I was using as the second AP in this setup before, a TL-WR841NDv9 with OpenWrt 19.07.6. There still seems to be a difference between max and current transmit power if I try to set it to 20 dBm, but it has an upper limit of 16 dBm.

All routers have the correct country code set to "HU - Hungary". I also tested country code set to "Driver Default" (doesn't change anything) and "00 - World" (same result). Setting it to US (the other code listed by iw) let's me set it to 18 dBm (and higher), but this isn't really a solution.

I tried a factory reset. I haven't tried using just the default firmware builds, since I'd like to long-term keep my custom images. If it helps, I can post the manifest for my custom image.

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