thats pretty normal with Ryzen, 10°C temp jumps. i have 1700X and have pretty much same thing and few of my friends who use that report the same. If it sits idle the cpu isnt used at all so temps go down fast. if you do something that pulls the cpu core for few seconds it will heat up pretty much instantly, thatswhy you get those.
Now depending what cooler and fan profile you have, it can be annoying when it randomly starts to pull the fans because of instant temp rise. I have my fan profile configured so that it doesnt even change the fan speed until its 55°C and next step 65°C and then 70°C. So it wont spin-up and down the fans constantly when doing some simple tasks.

so back to the point if the fan noise doesn't bother you then those numbers seem perfectly fine and nothing to worry about.

The thermal expansion happening in that 10-20C temperature change is probably not significant enough to affect the life expectancy of your CPU. If it worries you though, you can probably try and set up a fan curve that brings the ambient idle temperature closer to the maximum temperature, but it would likely be too much trouble for essentially no benefit.

As I recently changed my CPU (Xeon E3 1231 -> i7 4790k) I also monitored temperature for a few days. I was also surprised by the same results you are seeing, but after a little duckduckgo investigation I was asured that this is pretty standard with modern CPUs because the clock down if idle (my i7 goes down to 1200MHz in idle) and can instantly clock up as needed and then heat up instantly by about 20°C. As Xpander pointed out, adjusting your fan profile helps in keeping the noise down, and the temperature jumps are pretty normal.