The following applies to OpenTherm boilers!

After reaching the desired temperature, the status LED will sometimes flash red. This is because:

If it is colder outside than inside then the house is constantly losing heat to the environment. If we do not compensate that loss, the house will cool down. We want the most comfortable possible indoor temperature. Comfortable means sufficiently high and without peaking or dipping.

As we just stated, the house constantly loses heat to the environment. If we want to keep a constant room temperature, we have to continually top it up by turning on the boiler.

Of course, your boiler won't be on full, it burns at a power that generates enough heat to replace the heat loss.

The status LED is therefore red to maintain the house temperature and not to heat it further.