I have had covid (once) and about 5 mRNA COVID shots. Have not had the experience of sudden reduction of maximum or average HR. The hypothesis that an mRNA shot could have this effect strikes me as a wild extrapolation from very little data.

However, I do see seeming fluctuations in my average and maximum HR, and I have for at least 15 years. It is partly related to fitness and partly a mystery. I tend to think that my body has a low heart rate mode and a high heart rate mode. In the former, my average HR is lower for the same exertion or average power, and also I have trouble achieving my maximum HR, which has changed relative little since I started keeping track in my late 40s (I'm 64 now). In my high HR mode, my average HR is higher, and I can get my HR up to at or near my max (which is 181 BPM now, and was 184 15 years ago). In the low mode, I won't see much above 170, no matter how hard the effort, and mostly I stay below 168. Over periods of months, I go from one to another and back. The LHR mode tends to correspond to higher training volume, but not exactly.