The change in voltage is minimal so I assume that by using the radio you meant listening, not transmitting which uses significantly more power so would lower the voltage more quickly. However 12.6V doesn't seem low enough to warrant a low battery warning.
Edit. I wonder if the tuner is drawing excessive current which is causing the low voltage warning whilst tuning but the voltage recovers once tuning has ceased. Have you monitored the voltage whilst tuning? Maybe the tuner's drive is partly seized causing both excessive power consumption and lack of tuning sometimes.
MY12 RRV 4.4 TDV8 AB, +LLAMS, +e-diff, +ACC stop/go. Produce LLAMS for LR/RR, Jeep GC/Dodge Ram
VK2HFG and APRS W1 digi
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