What the incubators read can't be trusted.
The two best thermometers I know that won't break the bank are
https://www.brinsea.com/p-394-spot-check-digital-incubator-thermometer.aspx
I found another recently that is very good and you can monitor it on your smartphone.
It is a govee. It is the most accurate I know of.
https://www.govee.com/products/107/govee-bluetooth-temperature-humidity-monitor
I've never owned seramas but I imagine the tiny eggs are negatively affected because of the size and how fluctuating temperatures negatively affect the embryo. Larger eggs internal temperature will lag ambient temperature more.
IMO, if an incubator maintains more steady temperatures and keep them closer to 99.5 rather than fluctuating from 98.5 to 101.5 which is what many incubators do. With an instant read thermometer, like those I mentioned, you will see how frequently and extremely most incubators vacillate in temperature.