OK, I changed it.
To me, a spike is generally an unexplained, most likely false or undesired, short-term condition probably due directly to some variable physical phenomenon. I believe - until proven wrong - that what has been presented can be fully explained, as I have done. The original cause might be something else, but that of which you write is a deliberate, fixed, known, identified value sent by a DS18B20 sensor, and it has a meaning. It is visually presented by emonCMS as a ‘spike’.
If anyone is not interested in being aware that a reading has been missed, an 'If = ’ processing step in emonCMS should be considered.
The latest version of emonLibCM attempts to trap an invalid 85 °C and convert it to the BAD_TEMPERATURE value 30400 (304 °C), as documented.