Without looking at logs, I am sure you know, but all wideband me7.5 I've seen disables wideband lambda control after a certain threshold on WOT, never looked much into it but I am fairly sure it has to do with safety , guess you dont wanna make correction at WOT from a bad sensor/poisoined sensor. You need to check conditions as for what it is, egt , control limit over a period etc.
Sometimes it keeps last value too, there were discussions about this behaviour back then.
Oh yeah I forgot it also gets turned off with BTS... but if you wanna check actual conditions refer to above
Attached is a bad tune/bad sensor car but I've seen this behaviour multiple times, had a switch off at 3.5k or so cause of threshold exceeeding, then on top RPM it just set value even when lower read limit of the sensor is exceeded.
Graph you posted is trigered by LALIUSMN
Not my case
I give up from thinking what is wrong here, I will tell friend to fix this other issues on car and then will log again and retune it