I've updated to Marlin 2.0.7 when I set up a BLTouch and now it seems the temperature does not match the display. Before the update, I was printing with PLA filament at 205 °C and bed at 60 °C without issue. After the update, my prints are full of stringing, so I decided to lower the temperature until 180° and now it's fine. (everything above 185 °C is stringing) I've tried tuning PID but it wasn't better. So I assume my printer sees 205 °C but the real nozzle temp is higher.
When you have updated to Marlin, you were supposed to configure it carefully. This is maybe laborious and sometimes difficult procedure, but very important. Basic guidelines are described in Configuring Marlin official guide.
I assume that you obviously did it, at least in some part to enable BLTouch. You should review the Thermal Settings section and update it with valid thermistors types for bed and hotend in Configuration.h:
#define TEMP_SENSOR_0 1 ... #define TEMP_SENSOR_BED 1
The list of available predefined settings (numerical identifiers) is just above these
#define macros. The optimistic assumption is that you know that hardware details. Popular value is
1 for EPCOS 100k (older repraps) or NTC 3950 100k, but there are many examples of troubleshooting and advices on Internet. For more exotic temperature sensors you should set value
1000 and specify own details in Configuration_adv.h.
If all these settings are correct, then possibly your thermistor is faulty. There is always a chance of coincidence with some damage unrelated to last upgrades. You can follow these instructions to verify it.