I just wanted to share here the findings about a local repeater which works with many other RDA1846 radios (mostly the FM ones) but did not activate on both OpenMDUV380 and stock firmware in RT3s, DM-1701 and Ailunce HD1 versions (stock firmware only). Since it did work with DM-X (MD1702) stock firmware, I had a look with disassembler and PDF datasheet on the registers and found the culprit. Interestingly, after the localization of the problem, I also found out that switching the OpenMDUV380 into wide FM mode actually activated the repeater (but it would break channel width specified by IARU).
To make the long story short, the culprit is register 0x59 lowest 6 bits setting in hardware/AT1846S.c. The firmware has its value set to 0x10, but in my case at least 0x13 was required, with save margin 0x14 was chosen. In stock firmwares, this value can be typically calibrated separately for different TX frequencies and different CTCSS tone frequencies, or at least centrally set to a specific value. Maybe it could be included in radio calibration screen as well (but this would require changes in several places of the code)?
For now, replacing:
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{0x59, 0x0B, 0x90},
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{0x59, 0x0B, 0x94},